Fix VpnSubnetRouteHandler handling of getSubnetToDpn()

Fix VpnSubnetRouteHandler handling of getSubnetToDpn()

List<SubnetToDpn> may be null here:

java.lang.NullPointerException: null
        at org.opendaylight.netvirt.vpnmanager.VpnSubnetRouteHandler.onInterfaceUp(VpnSubnetRouteHandler.java:623) ~[?:?]
        at org.opendaylight.netvirt.vpnmanager.SubnetRouteInterfaceStateChangeListener.lambda$add$0(SubnetRouteInterfaceStateChangeListener.java:115) ~[?:?]
        at org.opendaylight.infrautils.utils.ClassLoaders.lambda$call$2(ClassLoaders.java:39) ~[293:org.opendaylight.infrautils.util:1.6.1]
        at org.opendaylight.infrautils.utils.ClassLoaders.call(ClassLoaders.java:47) ~[293:org.opendaylight.infrautils.util:1.6.1]
        at org.opendaylight.infrautils.utils.ClassLoaders.call(ClassLoaders.java:39) ~[293:org.opendaylight.infrautils.util:1.6.1]
        at org.opendaylight.infrautils.jobcoordinator.internal.JobCoordinatorImpl$MainTask.runWithUncheckedExceptionLogging(JobCoordinatorImpl.java:398) ~[?:?]
        at org.opendaylight.infrautils.utils.concurrent.LoggingUncaughtThreadDeathContextRunnable.run(LoggingUncaughtThreadDeathContextRunnable.java:60) ~[293:org.opendaylight.infrautils.util:1.6.1]
        at org.opendaylight.infrautils.utils.ClassLoaders.lambda$run$0(ClassLoaders.java:26) ~[293:org.opendaylight.infrautils.util:1.6.1]
        at org.opendaylight.infrautils.utils.ClassLoaders.call(ClassLoaders.java:47) ~[293:org.opendaylight.infrautils.util:1.6.1]
        at org.opendaylight.infrautils.utils.ClassLoaders.run(ClassLoaders.java:25) ~[293:org.opendaylight.infrautils.util:1.6.1]
        at org.opendaylight.infrautils.utils.ClassLoaders.lambda$wrap$3(ClassLoaders.java:54) ~[293:org.opendaylight.infrautils.util:1.6.1]
        at java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1402) [?:?]
        at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289) [?:?]
        at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056) [?:?]
        at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692) [?:?]
        at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157) [?:?]

This patch fixes it up in a slighly different way to what
Magnesium did.

JIRA: NETVIRT-1636
Change-Id: Id075318b144ff34e680e16bfb426aaaa4fbe6af9
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>