avcodec/msmpeg4dec: Check for cbpy VLC errors

Fixes: runtime error: left shift of negative value -1
Fixes: 1480/clusterfuzz-testcase-minimized-5188321007370240

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
