gdkwindow: Fix potential NULL pointer dereference

gdk_window_ensure_native() can end up with a NULL parent pointer, which
it passes to find_native_parent_above()…but that expects a non-NULL
parent.

Found with scan-build.

https://bugzilla.gnome.org/show_bug.cgi?id=712760

gdk/gdkwindow.c

@@ -1110,6 +1110,9 @@ find_native_sibling_above (GdkWindow *parent,
 {
   GdkWindow *w;
 
+  if (!parent)
+    return NULL;
+
   w = find_native_sibling_above_helper (parent, child);
   if (w)
     return w;