avcodec/hevc: Simplify get_qPy_pred()

Fixes use of uninitialized memory
Fixes: 93728afd9aa074ba14a09bfd93a632fd-asan_static-oob_124a17d_1445_cov_1021181966_DBLK_D_VIXS_1.bit
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
