Fix for NPE on port delete

Fix for NPE on port delete

2015-05-15 13:30:06,114 | WARN  | n-invoker-impl-0 | TransactionInvokerImpl           | 332 - org.opendaylight.ovsdb.southbound-impl - 1.1.0.SNAPSHOT | Exception invoking Transaction:
java.lang.NullPointerException
        at org.opendaylight.ovsdb.southbound.transactions.md.OvsdbPortRemoveCommand.execute(OvsdbPortRemoveCommand.java:53)[332:org.opendaylight.ovsdb.southbound-impl:1.1.0.SNAPSHOT]
                at org.opendaylight.ovsdb.southbound.transactions.md.OvsdbOperationalCommandAggregator.execute(OvsdbOperationalCommandAggregator.java:29)[332:org.opendaylight.ovsdb.southbound-impl:1.1.0.SNAPSHOT]
                        at org.opendaylight.ovsdb.southbound.transactions.md.TransactionInvokerImpl.run(TransactionInvokerImpl.java:77)[332:org.opendaylight.ovsdb.southbound-impl:1.1.0.SNAPSHOT]
                                at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)[:1.7.0_71]
                                        at java.util.concurrent.FutureTask.run(FutureTask.java:262)[:1.7.0_71]
                                                at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)[:1.7.0_71]
                                                        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)[:1.7.0_71]
                                                                at java.lang.Thread.run(Thread.java:745)[:1.7.0_71]
                                                                2015-05-15 13:30:06,115 | WARN  | n-invoker-impl-0 | TransactionInvokerImpl           | 332 - org.opendaylight.ovsdb.southbound-impl - 1.1.0.SNAPSHOT | Exception invoking Transaction:
                                                                java.lang.IllegalStateException: Previous transaction is not ready yet
                                                                        at com.google.common.base.Preconditions.checkState(Preconditions.java:173)[78:com.google.guava:18.0.0]
                                                                                at org.opendaylight.controller.cluster.datastore.TransactionChainProxy.checkReadyState(TransactionChainProxy.java:133)[266:org.opendaylight.controller.sal-distributed-datastore:1.2.0.SNAPSHOT]
                                                                                        at org.opendaylight.controller.cluster.datastore.TransactionChainProxy.allocateWriteTransaction(TransactionChainProxy.java:121)[266:org.opendaylight.controller.sal-distributed-datastore:1.2.0.SNAPSHOT]
                                                                                                at org.opendaylight.controller.cluster.datastore.TransactionChainProxy.newReadWriteTransaction(TransactionChainProxy.java:101)[266:org.opendaylight.controller.sal-distributed-datastore:1.2.0.SNAPSHOT]
                                                                                                        at org.opendaylight.controller.cluster.databroker.DOMBrokerReadWriteTransaction.createTransaction(DOMBrokerReadWriteTransaction.java:49)[266:org.opendaylight.controller.sal-distributed-datastore:1.2.0.SNAPSHOT]
                                                                                                                at org.opendaylight.controller.cluster.databroker.DOMBrokerReadWriteTransaction.createTransaction(DOMBrokerReadWriteTransaction.java:22)[266:org.opendaylight.controller.sal-distributed-datastore:1.2.0.SNAPSHOT]
                                                                                                                        at org.opendaylight.controller.cluster.databroker.AbstractDOMBrokerTransaction.getSubtransaction(AbstractDOMBrokerTransaction.java:57)[266:org.opendaylight.controller.sal-distributed-datastore:1.2.0.SNAPSHOT]
                                                                                                                                at org.opendaylight.controller.cluster.databroker.DOMBrokerReadWriteTransaction.read(DOMBrokerReadWriteTransaction.java:37)[266:org.opendaylight.controller.sal-distributed-datastore:1.2.0.SNAPSHOT]
                                                                                                                                        at org.opendaylight.controller.md.sal.binding.impl.AbstractForwardedTransaction.doRead(AbstractForwardedTransaction.java:63)[244:org.opendaylight.controller.sal-binding-broker-impl:1.2.0.SNAPSHOT]
                                                                                                                                                at org.opendaylight.controller.md.sal.binding.impl.BindingDOMReadWriteTransactionAdapter.read(BindingDOMReadWriteTransactionAdapter.java:31)[244:org.opendaylight.controller.sal-binding-broker-impl:1.2.0.SNAPSHOT]
                                                                                                                                                        at org.opendaylight.ovsdb.southbound.transactions.md.OvsdbBridgeUpdateCommand.readNode(OvsdbBridgeUpdateCommand.java:191)[332:org.opendaylight.ovsdb.southbound-impl:1.1.0.SNAPSHOT]
                                                                                                                                                                at org.opendaylight.ovsdb.southbound.transactions.md.OvsdbBridgeUpdateCommand.updateBridge(OvsdbBridgeUpdateCommand.java:83)[332:org.opendaylight.ovsdb.southbound-impl:1.1.0.SNAPSHOT]
                                                                                                                                                                        at org.opendaylight.ovsdb.southbound.transactions.md.OvsdbBridgeUpdateCommand.execute(OvsdbBridgeUpdateCommand.java:76)[332:org.opendaylight.ovsdb.southbound-impl:1.1.0.SNAPSHOT]
                                                                                                                                                                                at org.opendaylight.ovsdb.southbound.transactions.md.OvsdbOperationalCommandAggregator.execute(OvsdbOperationalCommandAggregator.java:29)[332:org.opendaylight.ovsdb.southbound-impl:1.1.0.SNAPSHOT]
                                                                                                                                                                                        at org.opendaylight.ovsdb.southbound.transactions.md.TransactionInvokerImpl.run(TransactionInvokerImpl.java:77)[332:org.opendaylight.ovsdb.southbound-impl:1.1.0.SNAPSHOT]
                                                                                                                                                                                                at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)[:1.7.0_71]
                                                                                                                                                                                                        at java.util.concurrent.FutureTask.run(FutureTask.java:262)[:1.7.0_71]
                                                                                                                                                                                                                at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)[:1.7.0_71]
                                                                                                                                                                                                                        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)[:1.7.0_71]
                                                                                                                                                                                                                                at java.lang.Thread.run(Thread.java:745)[:1.7.0_71]

Change-Id: I5a95a43faeefe4d91556dd1499cb0ba414974838
Signed-off-by: Sam Hague <shague@redhat.com>