check if column[i].button is valid before calling gtk_widget_intersect on

* gtk/gtkclist.c: check if column[i].button is valid before calling
gtk_widget_intersect on it in gtk_clist_draw

* gtk/gtkdnd.c: removed unused variable empty_bits

* gtk/gtkrc.c: we should use gslist_* functions with GSLists...

* gtk/gtktypeutils.[ch]: makes more send for gtk_type_is_a to return
a boolean (thanks to Elrond for noticing this)

-Yosh

gtk/gtkclist.c

@@ -4429,7 +4429,8 @@ gtk_clist_draw (GtkWidget    *widget,
 	{
 	  if (!clist->column[i].visible)
 	    continue;
-	  if (gtk_widget_intersect(clist->column[i].button, area, &child_area))
+	  if (clist->column[i].button &&
+	      gtk_widget_intersect(clist->column[i].button, area, &child_area))
 	    gtk_widget_draw (clist->column[i].button, &child_area);
 	}
     }