OpticalLinkProvider.processLink throws NPE

If an optical topo file is loaded before the linc switch connects, srcPort or dstPort can be null. At a minimum, we should guard against that. But perhaps, we should also queue these updates.

2014-12-07 01:30:27,574 | WARN | event-dispatch-0 | AbstractListenerRegistry | 74 - org.onosproject.onos-api - 1.0.0 | Exception encountered while processing event LinkEvent{time=2014-12-07T01:30:27.472, type=LINK_ADDED, subject=DefaultLink{src=ConnectPoint

{elementId=of:0000000000000006, portNumber=2}

, dst=ConnectPoint

{elementId=of:0000000000000002, portNumber=1}

, type=OPTICAL, state=ACTIVE, durable=true}}
java.lang.NullPointerException
at org.onosproject.optical.OpticalLinkProvider.processLink(OpticalLinkProvider.java:142)[163:org.onosproject.onos-app-optical:1.0.0]
at org.onosproject.optical.OpticalLinkProvider.access$400(OpticalLinkProvider.java:51)[163:org.onosproject.onos-app-optical:1.0.0]
at org.onosproject.optical.OpticalLinkProvider$InternalLinkListener.event(OpticalLinkProvider.java:111)[163:org.onosproject.onos-app-optical:1.0.0]
at org.onosproject.optical.OpticalLinkProvider$InternalLinkListener.event(OpticalLinkProvider.java:105)[163:org.onosproject.onos-app-optical:1.0.0]
at org.onosproject.event.AbstractListenerRegistry.process(AbstractListenerRegistry.java:63)[74:org.onosproject.onos-api:1.0.0]
at org.onosproject.event.impl.CoreEventDispatcher$DispatchLoop.run(CoreEventDispatcher.java:95)[141:org.onosproject.onos-core-net:1.0.0]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)[:1.8.0_25]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)[:1.8.0_25]
at java.lang.Thread.run(Thread.java:745)[:1.8.0_25]