NullPointerException when trying to add to a port a IPv6 subnet

whenever the subnet is of IPv6 we get NPE since the code assumes that the subnet is IPv4 only.

2016-05-21 08:33:55,940 | ERROR | on-dispatcher-51 | DataChangeListener | 180 - org.opendaylight.controller.sal-distributed-datastore - 1.4.0.SNAPSHOT | Error notifying listener org.opendaylight.netvirt.neutronvpn.Neutron
PortChangeListener
java.lang.NullPointerException
at org.opendaylight.netvirt.neutronvpn.NeutronPortChangeListener.addPortToSubnets(NeutronPortChangeListener.java:359)[324:org.opendaylight.netvirt.neutronvpn-impl:0.3.0.SNAPSHOT]
at org.opendaylight.netvirt.neutronvpn.NeutronPortChangeListener.handleNeutronPortUpdated(NeutronPortChangeListener.java:263)[324:org.opendaylight.netvirt.neutronvpn-impl:0.3.0.SNAPSHOT]
at org.opendaylight.netvirt.neutronvpn.NeutronPortChangeListener.update(NeutronPortChangeListener.java:168)[324:org.opendaylight.netvirt.neutronvpn-impl:0.3.0.SNAPSHOT]
at org.opendaylight.netvirt.neutronvpn.NeutronPortChangeListener.update(NeutronPortChangeListener.java:55)[324:org.opendaylight.netvirt.neutronvpn-impl:0.3.0.SNAPSHOT]
at org.opendaylight.genius.mdsalutil.AbstractDataChangeListener.updateData(AbstractDataChangeListener.java:86)[294:org.opendaylight.genius.mdsalutil-api:0.1.0.SNAPSHOT]
at org.opendaylight.genius.mdsalutil.AbstractDataChangeListener.onDataChanged(AbstractDataChangeListener.java:55)[294:org.opendaylight.genius.mdsalutil-api:0.1.0.SNAPSHOT]
at org.opendaylight.controller.md.sal.binding.impl.AbstractForwardedDataBroker$TranslatingDataChangeInvoker.onDataChanged(AbstractForwardedDataBroker.java:143)[151:org.opendaylight.controller.sal-binding-broker-impl:1.4.0.SNAPSHOT]
at org.opendaylight.controller.cluster.datastore.DataChangeListener.dataChanged(DataChangeListener.java:70)[180:org.opendaylight.controller.sal-distributed-datastore:1.4.0.SNAPSHOT]
at org.opendaylight.controller.cluster.datastore.DataChangeListener.handleReceive(DataChangeListener.java:42)[180:org.opendaylight.controller.sal-distributed-datastore:1.4.0.SNAPSHOT]
at org.opendaylight.controller.cluster.common.actor.AbstractUntypedActor.onReceive(AbstractUntypedActor.java:25)[175:org.opendaylight.controller.sal-clustering-commons:1.4.0.SNAPSHOT]
at akka.actor.UntypedActor$$anonfun$receive$1.applyOrElse(UntypedActor.scala:165)[163:com.typesafe.akka.actor:2.4.4]
at akka.actor.Actor$class.aroundReceive(Actor.scala:482)[163:com.typesafe.akka.actor:2.4.4]
at akka.actor.UntypedActor.aroundReceive(UntypedActor.scala:95)[163:com.typesafe.akka.actor:2.4.4]
at akka.actor.ActorCell.receiveMessage(ActorCell.scala:526)[163:com.typesafe.akka.actor:2.4.4]
at akka.actor.ActorCell.invoke(ActorCell.scala:495)[163:com.typesafe.akka.actor:2.4.4]
at akka.dispatch.Mailbox.processMailbox(Mailbox.scala:257)[163:com.typesafe.akka.actor:2.4.4]
at akka.dispatch.Mailbox.run(Mailbox.scala:224)[163:com.typesafe.akka.actor:2.4.4]
at akka.dispatch.Mailbox.exec(Mailbox.scala:234)[163:com.typesafe.akka.actor:2.4.4]
at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)[159:org.scala-lang.scala-library:2.11.8.v20160304-115712-1706a37eb8]
at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339)[159:org.scala-lang.scala-library:2.11.8.v20160304-115712-1706a37eb8]
at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)[159:org.scala-lang.scala-library:2.11.8.v20160304-115712-1706a37eb8]
at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)[159:org.scala-lang.scala-library:2.11.8.v20160304-115712-1706a37eb8]