Fix NPE in AclServiceUtils

When a VM is booted with a port that is having an IPv6 address in allowed_address_pairs the following exception is seen from AclServiceUtils.

2016-09-02 14:27:48,998 | INFO | pool-43-thread-1 | AbstractIngressAclServiceImpl | 350 - org.opendaylight.netvirt.aclservice-impl - 0.4.0.SNAPSHOT | programFixedRules : adding default rules.
Exception in thread "pool-43-thread-1" java.lang.NullPointerException
at org.opendaylight.netvirt.aclservice.utils.AclServiceUtils.buildIpMatches(AclServiceUtils.java:448)
at org.opendaylight.netvirt.aclservice.StatefulIngressAclServiceImpl.programConntrackRecircRules(StatefulIngressAclServiceImpl.java:118)
at org.opendaylight.netvirt.aclservice.StatefulIngressAclServiceImpl.programIngressAclFixedConntrackRule(StatefulIngressAclServiceImpl.java:181)
at org.opendaylight.netvirt.aclservice.StatefulIngressAclServiceImpl.programSpecificFixedRules(StatefulIngressAclServiceImpl.java:71)
at org.opendaylight.netvirt.aclservice.AbstractAclServiceImpl.programAclWithAllowedAddress(AbstractAclServiceImpl.java:170)
at org.opendaylight.netvirt.aclservice.AbstractAclServiceImpl.applyAcl(AbstractAclServiceImpl.java:78)
at org.opendaylight.netvirt.aclservice.AclServiceManagerImpl.notify(AclServiceManagerImpl.java:54)
at org.opendaylight.netvirt.aclservice.listeners.AclInterfaceStateListener.add(AclInterfaceStateListener.java:109)
at org.opendaylight.netvirt.aclservice.listeners.AclInterfaceStateListener.add(AclInterfaceStateListener.java:35)
at org.opendaylight.genius.datastoreutils.AsyncDataTreeChangeListenerBase$DataTreeChangeHandler.run(AsyncDataTreeChangeListenerBase.java:124)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
2016-09-02 14:27:51,997 | INFO | nPool-1-worker-5 | VpnInterfaceManager | 341 - org.opendaylight.netvirt.vpnmanager-impl - 0.4.0.SNAPSHOT | Binding vpn service to interface 4eb3e908-612d-482e-ba04-0b0cfb59c9bb