avcodec/hevcdec: constrained intra predict, do not check top left IS_INTRA if it's not available

fix ticket: 8932

For poc 2, we have tile boundary at x = 640.
When we predict cu(640,912),the top left pixel is not avaliable to the cu.
So, we can not check it's intra or not. We need set top[-1] = top[0] directly.
see 8.4.4.2.1 for details
Signed-off-by: Xu Guangxin <oddstone@gmail.com>
Signed-off-by: Linjie Fu <linjie.justin.fu@gmail.com>

libavcodec/hevcpred_template.c

@@ -214,7 +214,7 @@ do {                                  \
                 while (j < size_max_x && !IS_INTRA(j, -1))
                     j++;
                 if (j > 0)
-                    if (x0 > 0) {
+                    if (cand_up_left) {
                         EXTEND_LEFT_CIP(top, j, j + 1);
                     } else {
                         EXTEND_LEFT_CIP(top, j, j);