swscale/graph: make noop loop more robust

The current loop only works if the input and output have the same number
of planes. However, with the new scaling logic, we can also optimize into a
noop the case where the input has extra unneeded planes.

For the memcpy fallback to work in these cases we have to instead check if
the *output* pointer is set, rather than the input pointer.

Author: haasn

libswscale/graph.c

@@ -115,8 +115,10 @@ static void run_copy(const SwsImg *out_base, const SwsImg *in_base,
     SwsImg in  = shift_img(in_base,  y);
     SwsImg out = shift_img(out_base, y);
 
-    for (int i = 0; i < FF_ARRAY_ELEMS(in.data) && in.data[i]; i++) {
+    for (int i = 0; i < FF_ARRAY_ELEMS(out.data) && out.data[i]; i++) {
         const int lines = h >> vshift(in.fmt, i);
+        av_assert1(in.data[i]);
+
         if (in.linesize[i] == out.linesize[i]) {
             memcpy(out.data[i], in.data[i], lines * out.linesize[i]);
         } else {