avformat/mov: Check av_add_index_entry() return value

Fixes NULL pointer dereference
Fixes: signal_sigsegv_b060e0_3794_cov_1293954059_vc1-wmapro.ism

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