hevc: Bound check slice_qp

The T-REC-H.265-2013044 page 79 states they have to be into the range
[-s->sps->qp_bd_offset, 51].

Fixes: asan_stack-oob_eae8e3_9522_WP_MAIN10_B_Toshiba_3.bit
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
