avfilter/vf_gradfun: Do not overread last line

The code works in steps of 2 lines and lacks support for odd height
Implementing odd height support is better but for now this fixes the
out of array access

Fixes: out of array access
Fixes: tickets/10702/poc6ffmpe

Found-by: Zeng Yunxiang
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/vf_gradfun.c

@@ -92,7 +92,7 @@ static void filter(GradFunContext *ctx, uint8_t *dst, const uint8_t *src, int wi
     for (y = 0; y < r; y++)
         ctx->blur_line(dc, buf + y * bstride, buf + (y - 1) * bstride, src + 2 * y * src_linesize, src_linesize, width / 2);
     for (;;) {
-        if (y < height - r) {
+        if (y + 1 < height - r) {
             int mod = ((y + r) / 2) % r;
             uint16_t *buf0 = buf + mod * bstride;
             uint16_t *buf1 = buf + (mod ? mod - 1 : r - 1) * bstride;