Support pcm_mulaw in Hikvision CCTV mpeg program streams.

Fixes decoding the sample from videolan trac ticket #8344.
Fix avisynth_context_create constructor error handling.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Update MAINTAINERS with information about avisynth demuxer.

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