Fix NPE in NeutronvpnNatManager

On creating a Neutron network with IPv4 subnet and associating it with a Router, the following exception is logged.

opendaylight-user@root>log:exception-display
java.lang.NullPointerException
at org.opendaylight.netvirt.neutronvpn.NeutronvpnNatManager.externalFixedIpsChanged(NeutronvpnNatManager.java:168)[344:org.opendaylight.netvirt.neutronvpn-impl:0.4.0.SNAPSHOT]
at org.opendaylight.netvirt.neutronvpn.NeutronvpnNatManager.handleExternalNetworkForRouter(NeutronvpnNatManager.java:111)[344:org.opendaylight.netvirt.neutronvpn-impl:0.4.0.SNAPSHOT]
at org.opendaylight.netvirt.neutronvpn.NeutronRouterChangeListener.add(NeutronRouterChangeListener.java:76)[344:org.opendaylight.netvirt.neutronvpn-impl:0.4.0.SNAPSHOT]
at org.opendaylight.netvirt.neutronvpn.NeutronRouterChangeListener.add(NeutronRouterChangeListener.java:33)[344:org.opendaylight.netvirt.neutronvpn-impl:0.4.0.SNAPSHOT]
at org.opendaylight.genius.mdsalutil.AbstractDataChangeListener.createData(AbstractDataChangeListener.java:68)[304:org.opendaylight.genius.mdsalutil-api:0.2.0.SNAPSHOT]
at org.opendaylight.genius.mdsalutil.AbstractDataChangeListener.onDataChanged(AbstractDataChangeListener.java:54)[304:org.opendaylight.genius.mdsalutil-api:0.2.0.SNAPSHOT]
at org.opendaylight.controller.md.sal.binding.impl.AbstractForwardedDataBroker$TranslatingDataChangeInvoker.onDataChanged(AbstractForwardedDataBroker.java:143)[174:org.opendaylight.controller.sal-binding-broker-impl:1.5.0.SNAPSHOT]
at org.opendaylight.controller.cluster.datastore.DataChangeListener.dataChanged(DataChangeListener.java:71)[204:org.opendaylight.controller.sal-distributed-datastore:1.5.0.SNAPSHOT]
at org.opendaylight.controller.cluster.datastore.DataChangeListener.handleReceive(DataChangeListener.java:43)[204:org.opendaylight.controller.sal-distributed-datastore:1.5.0.SNAPSHOT]
at org.opendaylight.controller.cluster.common.actor.AbstractUntypedActor.onReceive(AbstractUntypedActor.java:26)[198:org.opendaylight.controller.sal-clustering-commons:1.5.0.SNAPSHOT]
at akka.actor.UntypedActor$$anonfun$receive$1.applyOrElse(UntypedActor.scala:165)[186:com.typesafe.akka.actor:2.4.7]
at akka.actor.Actor$class.aroundReceive(Actor.scala:484)[186:com.typesafe.akka.actor:2.4.7]
at akka.actor.UntypedActor.aroundReceive(UntypedActor.scala:95)[186:com.typesafe.akka.actor:2.4.7]
at akka.actor.ActorCell.receiveMessage(ActorCell.scala:526)[186:com.typesafe.akka.actor:2.4.7]
at akka.actor.ActorCell.invoke(ActorCell.scala:495)[186:com.typesafe.akka.actor:2.4.7]
at akka.dispatch.Mailbox.processMailbox(Mailbox.scala:257)[186:com.typesafe.akka.actor:2.4.7]
at akka.dispatch.Mailbox.run(Mailbox.scala:224)[186:com.typesafe.akka.actor:2.4.7]
at akka.dispatch.Mailbox.exec(Mailbox.scala:234)[186:com.typesafe.akka.actor:2.4.7]
at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)[182:org.scala-lang.scala-library:2.11.8.v20160304-115712-1706a37eb8]
at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339)[182:org.scala-lang.scala-library:2.11.8.v20160304-115712-1706a37eb8]
at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)[182:org.scala-lang.scala-library:2.11.8.v20160304-115712-1706a37eb8]
at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)[182:org.scala-lang.scala-library:2.11.8.v20160304-115712-1706a37eb8]