patch 8.2.0559: clearing a struct is verbose

Problem:    Clearing a struct is verbose.
Solution:   Define and use CLEAR_FIELD() and CLEAR_POINTER().
diff --git a/src/gui_x11.c b/src/gui_x11.c
index ce619f6..f05de7b 100644
--- a/src/gui_x11.c
+++ b/src/gui_x11.c
@@ -2267,7 +2267,7 @@
 	return (guicolor_T)available.pixel;
 #endif
     colormap = DefaultColormap(gui.dpy, DefaultScreen(gui.dpy));
-    vim_memset(&available, 0, sizeof(XColor));
+    CLEAR_FIELD(available);
     available.red = r << 8;
     available.green = g << 8;
     available.blue = b << 8;