avformat/mov: Check dimensions before setting aspect

Fixes division by 0
Fixes: asan_heap-uaf_143f420_728_cov_1441472032_HDV_0113.MP4

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
