Fixes: signed integer overflow: -9223372036854775808 - 17 cannot be represented in type 'long'
Fixes: 18768/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5674385247830016

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