avcodec/mpeg12dec: Fix runtime error: left shift of negative value -2

671/clusterfuzz-testcase-4990381827555328

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg


Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>