avformat/mov: Check ctts_count before use

Fixes out of array read
Fixes: asan_heap-oob_ae74b5_3610_cov_1739568095_test.3g2

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