avformat/mov: Check STSD atom more completely

Fixes out of array read
Fixes: 13262c363a28da8d6bdcc472aed6e9dc/asan_heap-oob_cfb5e2_3733_31cf3fcc783295c34222eb070a784f84.3gp

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
