Fixes: signed integer overflow: -2147483648 - 1 cannot be represented in type 'int'
Fixes: 15568/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DIRAC_fuzzer-5634719611355136

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: NPaul B Mahol <onemda@gmail.com>
Signed-off-by: NMichael Niedermayer <michael@niedermayer.cc>