Merge commit '35c85806'

* commit '35c85806':
  avconv: stop using AVStream.codec

Not merged, as ffmpeg.c is not migrated to codecpar yet, and this commit
only changes minor parts of AVStream.codec usage, which should get applied
after ffmpeg.c is using codecpar.
Merged-by: Hendrik Leppkes <h.leppkes@gmail.com>