hevc: Bound check slice_qp

The T-REC-H.265-2013044 page 79 states it has to be in the range
[-s->sps->qp_bd_offset, 51].

Sample-Id: 00001386-google
Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
