Fixes: signed integer overflow: -2147479324 + -32568 cannot be represented in type 'int'
Fixes: 20103/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_GREMLIN_DPCM_fuzzer-5667667579240448

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