avutil/imgutils: don't fill data pointers for missing planes

The size for a previous plane doesn't signal the presence of another after it.
If the plane is present, av_image_fill_plane_sizes() will have returned a size
for it.

Fixes a regression since 3a8e927176.

Reported-by: Imad R. Faiad <irfaiad@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>

libavutil/imgutils.c

@@ -167,7 +167,7 @@ int av_image_fill_pointers(uint8_t *data[4], enum AVPixelFormat pix_fmt, int hei
     memset(data , 0, sizeof(data[0])*4);
 
     data[0] = ptr;
-    for (i = 1; i < 4 && sizes[i - 1] > 0; i++)
+    for (i = 1; i < 4 && sizes[i]; i++)
         data[i] = data[i - 1] + sizes[i - 1];
 
     return ret;