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>

libavcodec/hevcpred_template.c

@@ -155,6 +155,7 @@ static void FUNC(intra_pred)(HEVCContext *s, int x0, int y0, int log2_size, int
             left[i] = 128;
             top[i]  = 128;
         }
+        top[-1] = 128;
     }
     if (cand_bottom_left) {
         for (i = size + bottom_left_size; i < (size << 1); i++)