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_gtk_f.c b/src/gui_gtk_f.c
index ff9aee8..2a9e5a0 100644
--- a/src/gui_gtk_f.c
+++ b/src/gui_gtk_f.c
@@ -223,7 +223,7 @@
     {
 	GtkTypeInfo form_info;
 
-	vim_memset(&form_info, 0, sizeof(form_info));
+	CLEAR_FIELD(form_info);
 	form_info.type_name = "GtkForm";
 	form_info.object_size = sizeof(GtkForm);
 	form_info.class_size = sizeof(GtkFormClass);