avcodec/hevc: clear tab_slice_address in hevc_frame_start()

Fixes inconsistencies
Fixes use of uninitilaized memory
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>
