Fixes: signed integer overflow: 9223372036854775807 + 1 cannot be represented in type 'long'
Fixes: 16022/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5759796759756800

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