Fixes: runtime error: signed integer overflow: 623487 * 536870912 cannot be represented in type 'int'
Fixes: 3594/clusterfuzz-testcase-minimized-4650622935629824

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