Fixes: signed integer overflow: 9223372036854550860 + 530259564 cannot be represented in type 'long'
Fixes: 49093/clusterfuzz-testcase-minimized-ffmpeg_dem_ASF_O_fuzzer-4697179192688640

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