Fixes: signed integer overflow: 46802 * -71230 cannot be represented in type 'int'
Fixes: 4756/clusterfuzz-testcase-minimized-4812495563784192

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