avcodec/hevc: hls_decode_entry: check that the previous slice segment is available before decoding the next

Fixes use of uninitialized memory
Fixes out of array read
Fixes assertion failure
Fixes part of cb307d24befbd109c6f054008d6777b5/asan_static-oob_124a175_1445_cov_2355279992_DBLK_D_VIXS_1.bit
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>