Fixes: signed integer overflow: -144876608 * 16 cannot be represented in type 'int'
Fixes: 22782/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_MPEG4_fuzzer-6039584977977344

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit e361785ee05cc75d3caacf2f254160b0336f5358)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>