Fixes: 62276/clusterfuzz-testcase-minimized-ffmpeg_dem_MOV_fuzzer-4802790784303104
Fixes: signed integer overflow: 1768972133 + 968491058 cannot be represented in type 'int'

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