avfilter/vf_delogo: Use AVPixFmtDescriptor.nb_components

Relying on AVPixFmtDescriptor.nb_components is cleaner and faster than
checking data and linesize for every possible plane.
Signed-off-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/vf_delogo.c

@@ -256,7 +256,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *in)
     if (!sar.num)
         sar.num = sar.den = 1;
 
-    for (plane = 0; plane < 4 && in->data[plane] && in->linesize[plane]; plane++) {
+    for (plane = 0; plane < desc->nb_components; plane++) {
         int hsub = plane == 1 || plane == 2 ? hsub0 : 0;
         int vsub = plane == 1 || plane == 2 ? vsub0 : 0;