Bug 7124 - legacy netvirt: null pointer exception in SouthboundHandler

Bug 7124 - legacy netvirt: null pointer exception in SouthboundHandler

This patch avoids null pointer exception as follows.
With openstack tempest CI, the following exception occurs.

> java.lang.NullPointerException
>         at org.opendaylight.netvirt.openstack.netvirt.SouthboundHandlercd.processPortDelete(SouthboundHandler.java:184)[289:org.opendaylight.netvirt.openstack.net-virt:1.3.1.SNAPSHOT]
>         at org.opendaylight.netvirt.openstack.netvirt.SouthboundHandler.processPortEvent(SouthboundHandler.java:332)[289:org.opendaylight.netvirt.openstack.net-virt:1.3.1.SNAPSHOT]
>         at org.opendaylight.netvirt.openstack.netvirt.SouthboundHandler.processEvent(SouthboundHandler.java:274)[289:org.opendaylight.netvirt.openstack.net-virt:1.3.1.SNAPSHOT]
>         at org.opendaylight.netvirt.openstack.netvirt.impl.EventDispatcherImpl.dispatchEvent(EventDispatcherImpl.java:96)[289:org.opendaylight.netvirt.openstack.net-virt:1.3.1.SNAPSHOT]
>         at org.opendaylight.netvirt.openstack.netvirt.impl.EventDispatcherImpl.lambda$start$0(EventDispatcherImpl.java:58)[289:org.opendaylight.netvirt.openstack.net-virt:1.3.1.SNAPSHOT]
>         at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)[:1.8.0_111]
>         at java.util.concurrent.FutureTask.run(FutureTask.java:266)[:1.8.0_111]
>         at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)[:1.8.0_111]
>         at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)[:1.8.0_111]
>         at java.lang.Thread.run(Thread.java:745)[:1.8.0_111]

Change-Id: I8549fb799ac0ae797f4b84dc8d910ef02b65a0ad
Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>