vici: Ignore NULL message in raise_event()

There are a lot of calls like this:

  this->dispatcher->raise_event(this->dispatcher, "...", 0,
                                b->finalize(b));

However, if finalize() fails, e.g. because a previous call to add()
failed due to the size limit, it returns NULL.  This then caused a
segmentation fault in raise_event() when it interacted with that value.

Closes strongswan/strongswan#1278

src/libcharon/plugins/vici/suites/test_event.c

@@ -60,6 +60,9 @@ START_TEST(test_event)
 	ck_assert(vici_register(conn, "test", event_cb, &count) == 0);
 	ck_assert(vici_register(conn, "nonexistent", event_cb, &count) != 0);
 
+	/* should just get ignored */
+	dispatcher->raise_event(dispatcher, "test", 0, NULL);
+
 	dispatcher->raise_event(dispatcher, "test", 0, vici_message_create_from_args(
 		 VICI_KEY_VALUE, "key1", chunk_from_str("value1"),
 		VICI_END));

src/libcharon/plugins/vici/vici_dispatcher.c

@@ -497,6 +497,11 @@ METHOD(vici_dispatcher_t, raise_event, void,
 	event_t *event;
 	u_int *current;
 
+	if (!message)
+	{
+		return;
+	}
+
 	this->mutex->lock(this->mutex);
 	event = this->events->get(this->events, name);
 	if (event)