Apply the same logic used for subtitle stream selection to audio and video  stream selection.
Apply the same logic used for subtitle stream selection to audio and video 
stream selection.

Make the counting starts from 0, and disable the corresponding playbck
when the selected stream number is negative.

See the thread:
" [FFmpeg-devel] [PATCH] Add documentation for -ast, -vst, -sst".


git-svn-id: a4d7c1866f8397a4106e0b57fc4fbf792bbdaaaf@17693 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

prajnashi/ffmpeg,prajnashi/ffmpeg,prajnashi/ffmpeg,prajnashi/ffmpeg