avformat: add av_stream_get_parser() to access avformat AVParser

The AVStream.parser field is considered private and its location cannot be
preserved while preserving also ABI compatibility to libav, as libav added fields
before it.
Some tools like ffmpeg.c access this field though

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
ffmpeg: Use av_stream_get_parser() to avoid ABI issues

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>