avformat/bink: Check return value of av_add_index_entry()

Fixes null pointer dereference
Fixes: cdbf15cbd0a27cee958dd0b8800e452e-signal_sigsegv_737991_2083_cov_317652874_LBSTART.BIK
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
