avcodec/mdec: Fix runtime error: left shift of negative value -127

Fixes undefined behavior
Fixes: 1275/clusterfuzz-testcase-minimized-6718162017976320

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