Don't allow multiple decodes of the same ffmpeg audio stream. Because ffmpeg mixes...

Don't allow multiple decodes of the same ffmpeg audio stream. Because ffmpeg mixes stream reader state with decoder state we only have one decoder instance for the streamso multiple decode requests will garble the audio and/or cause an abort.
(Since the Universal preset specifies AC-3 passthru, using it on non-ac3 audio caused a second decode of the first audio track to be requested which triggered this bug and resulted in lots of aborts for wmv & avi conversions.)

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@1969 b64f7644-9d1e-0410-96f1-a4d463321fa5