avcodec/mpeg4videodec: forward return code in ff_mpeg4_decode_picture_header()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/mpeg4videodec: replace some return -1 by more specific error codes

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/mpeg4video_parser: fix spurious extradata parse warnings

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