avcodec/mpegvideo: Fix off-by-one error when decoding >8 bit MPEG-4

Fixes visual corruptions on two macroblocks from two frames from
https://samples.ffmpeg.org/ffmpeg-bugs/trac/ticket4447/A003C003_SR_422_23.98p.mxf

Reviewed-by: Kieran Kunhya <kierank@obe.tv>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
