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 ?