avcodec/hevc_ps: Check num_long_term_ref_pics_sps

      Fixes out of array access
Fixes: signal_sigsegv_35bd0f0_1182_cov_791726764_STRUCT_B_Samsung_4.bit
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>