Fixes: runtime error: signed integer overflow: -1408475220 + -1408475220 cannot be represented in type 'int'
Fixes: 3336/clusterfuzz-testcase-minimized-5656839179993088

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