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).
