avcodec/hevcpred_template: also initialize top[-1] for constrained intra prediction

Fixes out of array read
Fixes: 387713a12dc5cfa27fcb4178084ce1ea-asan_stack-oob_131176a_1182_cov_3861068719_CAINIT_C_SHARP_3.bit
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>