Fixes: runtime error: signed integer overflow: 8903997421129740175 + 354481484684609529 cannot be represented in type 'long'
Fixes: 2045/clusterfuzz-testcase-minimized-6751255865065472

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