fade: fix draw_slice() check on fade->factor value

draw_slice() checks that the fade factor is < 65536 and only
calculates the fade if so. But the fade factor is clipped in
end_frame() by av_clip_uint16() to 65535, so the fade is calculated
for every frame.  This patch alters the check so that it compares with
< 65535 (UINT16_MAX).
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavfilter/vf_fade.c

@@ -101,7 +101,7 @@ static void draw_slice(AVFilterLink *inlink, int y, int h, int slice_dir)
     uint8_t *p;
     int i, j, plane;
 
-    if (fade->factor < 65536) {
+    if (fade->factor < UINT16_MAX) {
         /* luma or rgb plane */
         for (i = 0; i < h; i++) {
             p = outpic->data[0] + (y+i) * outpic->linesize[0];