Fix NPE on route delete

Fix NPE on route delete

Local testing has uncovered this:

2015-04-14 16:34:51,211 | INFO  | upCloseable-3-11 | BGPSessionImpl                   | 209 - org.opendaylight.bgpcep.bgp-rib-impl - 0.4.0.SNAPSHOT | Closing session: BGPSessionImpl{channel=[id: 0x0ff0457c, /127.0.0.2:36180 :> /127.0.0.1:179], state=IDLE}
2015-04-14 16:36:03,441 | ERROR | lt-dispatcher-16 | DataTreeChangeListenerActor      | 202 - org.opendaylight.controller.sal-distributed-datastore - 1.2.0.SNAPSHOT | Error notifying listener org.opendaylight.protocol.bgp.rib.impl.LocRibWriter@5917453f
java.lang.NullPointerException
        at org.opendaylight.protocol.bgp.rib.impl.LocRibWriter.onDataTreeChanged(LocRibWriter.java:183)[209:org.opendaylight.bgpcep.bgp-rib-impl:0.4.0.SNAPSHOT]
        at org.opendaylight.controller.cluster.datastore.DataTreeChangeListenerActor.dataChanged(DataTreeChangeListenerActor.java:53)[202:org.opendaylight.controller.sal-distributed-datastore:1.2.0.SNAPSHOT]

Which turns out to be a failure to check for deleted entry.

Change-Id: Ib4e0be69bbafdaf3e29698395a85224fdfa6c347
Signed-off-by: Robert Varga <rovarga@cisco.com>