Fixes: runtime error: signed integer overflow: 2147421862 - -33624063 cannot be represented in type 'int'
Fixes: 12885/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_H264_fuzzer-5733516975800320

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