vf_unsharp: fix out-of-buffer read

In apply_unsharp(), when y is >= height, prevent out-of-buffer reading
from src, read from the last buffer line in src2 instead.

The check was implemented in the original unsharp libmpcodecs code and
lost in the port.

This also fixes output discrepancy between the two filters.

Signed-off-by: Anton Khirnov <anton@khirnov.net>
(cherry picked from commit 998e8519ef)

libavfilter/vf_unsharp.c

@@ -70,6 +70,7 @@ static void unsharpen(uint8_t *dst, const uint8_t *src, int dst_stride, int src_
 
     int32_t res;
     int x, y, z;
+    const uint8_t *src2;
 
     if (!fp->amount) {
         if (dst_stride == src_stride)
@@ -84,9 +85,12 @@ static void unsharpen(uint8_t *dst, const uint8_t *src, int dst_stride, int src_
         memset(sc[y], 0, sizeof(sc[y][0]) * (width + 2 * fp->steps_x));
 
     for (y = -fp->steps_y; y < height + fp->steps_y; y++) {
+        if (y < height)
+            src2 = src;
+
         memset(sr, 0, sizeof(sr[0]) * (2 * fp->steps_x - 1));
         for (x = -fp->steps_x; x < width + fp->steps_x; x++) {
-            tmp1 = x <= 0 ? src[0] : x >= width ? src[width-1] : src[x];
+            tmp1 = x <= 0 ? src2[0] : x >= width ? src2[width-1] : src2[x];
             for (z = 0; z < fp->steps_x * 2; z += 2) {
                 tmp2 = sr[z + 0] + tmp1; sr[z + 0] = tmp1;
                 tmp1 = sr[z + 1] + tmp2; sr[z + 1] = tmp2;