Null checking in packet-out mechanism.

If the switch has recently disconnected, we can get some NullPointerExceptions.

Change-Id: Ia5a854ed5cd6a933974c855e7297faa542626d9e

@@ -97,7 +97,13 @@ implements PacketService, PacketProviderRegistry {
 
     private void localEmit(OutboundPacket packet) {
         final Device device = deviceService.getDevice(packet.sendThrough());
+
+        if (device == null) {
+            return;
+        }
+
         final PacketProvider packetProvider = getProvider(device.providerId());
+
         if (packetProvider != null) {
             packetProvider.emit(packet);
         }

@@ -95,6 +95,10 @@ public class DistributedPacketStore
         NodeId myId = clusterService.getLocalNode().id();
         NodeId master = mastershipService.getMasterFor(packet.sendThrough());
 
+        if (master == null) {
+            return;
+        }
+
         if (myId.equals(master)) {
             notifyDelegate(new PacketEvent(Type.EMIT, packet));
             return;