bus: do not leak message on invalid handler return value

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/6124>

subprojects/gstreamer/gst/gstbus.c

@@ -436,6 +436,7 @@ gst_bus_post (GstBus * bus, GstMessage * message)
     }
     default:
       g_warning ("invalid return from bus sync handler");
+      gst_message_unref (message);
       break;
   }
   return TRUE;