Fixes: runtime error: signed integer overflow: -2146071175 + -268479557 cannot be represented in type 'int'
Fixes: 5237/clusterfuzz-testcase-minimized-4569895275593728

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