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>