Fixes: signed integer overflow: 9223372036854775807 - -1 cannot be represented in type 'long'
Fixes: 23167/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-6425051741290496

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 9a42a67c5ca198a3879b7f3663cc44ccbcaf0bd3)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>