avformat/mov: Check next offset in mov_read_dref()

Fixes: signed integer overflow: 9223372036200463215 + 1109914409 cannot be represented in type 'long'
Fixes: 41480/clusterfuzz-testcase-minimized-ffmpeg_dem_MOV_fuzzer-6553086177443840

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg


Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>