winegstreamer: Dereference messages dropped from watch_bus().

From GStreamer documentation: "If the handler returns GST_BUS_DROP, it should
unref the message, else the message should not be unreffed by the sync handler."

Signed-off-by: Zebediah Figura <z.figura12@gmail.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>

dlls/winegstreamer/gstdemux.c

@@ -1280,6 +1280,7 @@ static GstBusSyncReply watch_bus(GstBus *bus, GstMessage *msg, gpointer data)
     default:
         break;
     }
+    gst_message_unref(msg);
     return GST_BUS_DROP;
 }