Fixes: signed integer overflow: -2142516591 + -267814575 cannot be represented in type 'int'
Fixes: 14450/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_H264_fuzzer-5716105319940096

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 4896fa18)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>