Fixes: signed integer overflow: 9079256848778919936 - -288230376151711746 cannot be represented in type 'long'
Fixes: 58248/clusterfuzz-testcase-minimized-ffmpeg_dem_OGG_fuzzer-6326851353313280

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