Likely a tighter check can be done
Fixes: signed integer overflow: 3305606804154370442 * 8 cannot be represented in type 'long'
Fixes: 70449/clusterfuzz-testcase-minimized-ffmpeg_dem_WEBM_DASH_MANIFEST_fuzzer-4771166007918592
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>