avcodec/hevc_ps: Check diff_cu_qp_delta_depth

Fixes undefined behavior
Fixes: asan_static-oob_17aa046_582_cov_1577759978_DBLK_G_VIXS_1.bit
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
