aout: rework reset before aout->stop
An asynchronous timing report would fail/assert after a stream_Reset().
This issue was hidden since, most of the time, vlc_aout_stream_Flush() is called before vlc_aout_stream_Delete() but there are some rare case where vlc_aout_stream_Delete() is called without a flush.
A simpler solution would be to always flush from vlc_aout_stream_Delete(), what do you think ?