Fixes: signed integer overflow: -1625810908 - 582229060 cannot be represented in type 'int'
Fixes: 10977/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SHORTEN_fuzzer-5732602018267136

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 2f888771)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>