avcodec/vc1_block: Fix mqaunt check for negative values

Fixes: out of array access
Fixes: ffmpeg_bof_4.avi
Fixes: ffmpeg_bof_5.avi
Fixes: ffmpeg_bof_6.avi

Found-by: Thuan Pham, Marcel Böhme, Andrew Santosa and Alexandru Razvan Caciulescu with AFLSmart
Reviewed-by: Jerome Borsboom <jerome.borsboom@carpalis.nl>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>