Gstreamer: Fix leak in prepare-window-handle message

If the handler returns GST_BUS_DROP, it should unref the message,
else the message should not be unreffed by the sync handler.

The message is not going to be unreffed after handler call is made.

Task-number: QTBUG-64142
Change-Id: Ib44f574e95457cdfab66b390c691a8b572987d66
Reviewed-by: Christian Stromme <christian.stromme@qt.io>

src/gsttools/qgstreamerbushelper.cpp

@@ -143,8 +143,10 @@ static GstBusSyncReply syncGstBusFilter(GstBus* bus, GstMessage* message, QGstre
     QMutexLocker lock(&d->filterMutex);
 
     for (QGstreamerSyncMessageFilter *filter : qAsConst(d->syncFilters)) {
-        if (filter->processSyncMessage(QGstreamerMessage(message)))
+        if (filter->processSyncMessage(QGstreamerMessage(message))) {
+            gst_message_unref(message);
             return GST_BUS_DROP;
+        }
     }
 
     return GST_BUS_PASS;