Fixes: runtime error: signed integer overflow: 1939661764 - -454942263 cannot be represented in type 'int'
Fixes: 3191/clusterfuzz-testcase-minimized-5688798451073024

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