Fixes: signed integer overflow: 2314885530818453566 + 7503032301549264928 cannot be represented in type 'long'
Fixes: 26639/clusterfuzz-testcase-minimized-ffmpeg_dem_ASF_O_fuzzer-6024222100684800
Alternatively this could be ignored but then the end condition of the loop
would be hard to reach as avio_tell() is int64_t
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>