Exception in VpnInterfaceManager for IPv6 subnets

Commit I1e6d60982e0d474c33cfa89a90737f2e7735f1a3 implements Arp
responder for internal subnet GW addresses. The implementation
assumes only IPv4 subnets and was not validated aganist IPv6 subnets.
When an IPv6 subnet is created, we notice the following exception.

2016-12-01 13:34:56,998 | ERROR | nPool-1-worker-3 | DataStoreJobCoordinator | 308 - org.opendaylight.genius.mdsalutil-api - 0.1.2.SNAPSHOT | Exception when executing jobEntry: JobEntry

{key='VPNINTERFACE-ece6b85b-b2d9-4536-9ed8-94e00e213243', mainWorker=org.opendaylight.netvirt.vpnmanager.VpnInterfaceManager$1@8176107, rollbackWorker=null, retryCount=0, futures=null}

java.lang.NullPointerException
at org.opendaylight.netvirt.vpnmanager.VpnInterfaceManager.getVpnSubnetGatewayIp(VpnInterfaceManager.java:1276)[344:org.opendaylight.netvirt.vpnmanager-impl:0.3.2.SNAPSHOT]
at org.opendaylight.netvirt.vpnmanager.VpnInterfaceManager.addArpResponderFlow(VpnInterfaceManager.java:1240)[344:org.opendaylight.netvirt.vpnmanager-impl:0.3.2.SNAPSHOT]
at org.opendaylight.netvirt.vpnmanager.VpnInterfaceManager.processVpnInterfaceAdjacencies(VpnInterfaceManager.java:679)[344:org.opendaylight.netvirt.vpnmanager-impl:0.3.2.SNAPSHOT]
at org.opendaylight.netvirt.vpnmanager.VpnInterfaceManager.processVpnInterfaceUp(VpnInterfaceManager.java:364)[344:org.opendaylight.netvirt.vpnmanager-impl:0.3.2.SNAPSHOT]
at org.opendaylight.netvirt.vpnmanager.VpnInterfaceManager$1.call(VpnInterfaceManager.java:240)[344:org.opendaylight.netvirt.vpnmanager-impl:0.3.2.SNAPSHOT]
at org.opendaylight.netvirt.vpnmanager.VpnInterfaceManager$1.call(VpnInterfaceManager.java:234)[344:org.opendaylight.netvirt.vpnmanager-impl:0.3.2.SNAPSHOT]
at org.opendaylight.genius.datastoreutils.DataStoreJobCoordinator$MainTask.run(DataStoreJobCoordinator.java:251)[308:org.opendaylight.genius.mdsalutil-api:0.1.2.SNAPSHOT]
at java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1402)[:1.8.0_111]
at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)[:1.8.0_111]
at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)[:1.8.0_111]
at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)[:1.8.0_111]
at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)[:1.8.0_111]