avcodec/mdec: Fix signed integer overflow: 28835400 * 83 cannot be represented in type 'int'

Fixes: 1346/clusterfuzz-testcase-minimized-5776732600664064

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