avcodec/hevc: Check entry point arrays for malloc failure

Fixes null pointer dereference
Fixes: signal_sigsegv_e1d3b6_2192_DBLK_F_VIXS_2.bit
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
