Fixes: signed integer overflow: 9223372036854775584 + 536870912 cannot be represented in type 'long'
Fixes: 55844/clusterfuzz-testcase-minimized-ffmpeg_dem_MOV_fuzzer-510613920664780
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 53c1f5c2e2)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>