Fixes: signed integer overflow: -2147483648 - 5 cannot be represented in type 'int'
Fixes: 54242/clusterfuzz-testcase-minimized-ffmpeg_BSF_DTS2PTS_fuzzer-472928339243827

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