message: protect against null message sources

Message sources can be null, check if it is before trying to
access its name.

gst/gstmessage.c

@@ -192,8 +192,7 @@ gst_message_finalize (GstMessage * message)
   g_return_if_fail (message != NULL);
 
   GST_CAT_LOG (GST_CAT_MESSAGE, "finalize message %p, %s from %s", message,
-      GST_MESSAGE_TYPE_NAME (message),
-      GST_OBJECT_NAME (GST_MESSAGE_SRC (message)));
+      GST_MESSAGE_TYPE_NAME (message), GST_MESSAGE_SRC_NAME (message));
 
   if (GST_MESSAGE_SRC (message)) {
     gst_object_unref (GST_MESSAGE_SRC (message));