Fixes: signed integer overflow: 1 - -9223372036854775808 cannot be represented in type 'long'
Fixes: 23490/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5133490093031424

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegReviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit a8fb7612a97530bdd0b2549dacf91dcf71a3187a)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>