libovccclient/ovccclient-server: send_message: ensure message is non-null

Even if the Message parameter is non-nullable, in case of problem creating the
Message instance we still may receive a null parameter here. This triggers
an assert failure in send_loop_queue.push without error so we get a nice
deadlock waiting for a non-queued message to be sent...

libovccclient/src/ovccclient-server.vala

@@ -277,6 +277,8 @@ namespace OVCCClient
     {
       if (AtomicInt.get (ref send_loop_running) == 0) {
         throw new ServerError.COMMUNICATION_FAILED ("Send loop is not running");
+      } else if (msg == null) { /* send_loop_queue.push needs non-null */
+        throw new ServerError.COMMUNICATION_FAILED ("Message to send is null");
       } else {
         ulong         id = 0;
         ulong         cancel_id = 0;