avcodec/mpeg4videodec: Fix runtime error: signed integer overflow: -135088512 * 16 cannot be represented in type 'int'

Fixes: 736/clusterfuzz-testcase-5580263943831552

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