fade: make draw_slice() chroma check against planes 1 and 2

draw_slice() checks that planes 0 and 1 of AVFilterBufferRef's data
are not NULL before manipulating planes 1 and 2. This patch makes the
check against planes 1 and 2. More senseful and possibly more robust.

libavfilter/vf_fade.c

@@ -114,7 +114,7 @@ static void draw_slice(AVFilterLink *inlink, int y, int h, int slice_dir)
             }
         }
 
-        if (outpic->data[0] && outpic->data[1]) {
+        if (outpic->data[1] && outpic->data[2]) {
             /* chroma planes */
             for (plane = 1; plane < 3; plane++) {
                 for (i = 0; i < h; i++) {