combobox: Avoid a pointless assignment

Don’t get the active item pointer before the grid/non-grid conditional,
because if we’re in grid mode, we re-get it before selecting it anyway.

gtk/gtkcombobox.c

@@ -2153,8 +2153,6 @@ gtk_combo_box_menu_popup (GtkComboBox    *combo_box,
 
   g_object_set (priv->popup_widget, "menu-type-hint", GDK_WINDOW_TYPE_HINT_COMBO, NULL);
 
-  active = gtk_menu_get_active (GTK_MENU (priv->popup_widget));
-
   if (priv->wrap_width > 0 || priv->cell_view == NULL)
     {
       gtk_css_gadget_get_border_allocation (priv->gadget, &border_allocation, NULL);
@@ -2180,6 +2178,8 @@ gtk_combo_box_menu_popup (GtkComboBox    *combo_box,
       GList *i;
       GtkWidget *child;
 
+      active = gtk_menu_get_active (GTK_MENU (priv->popup_widget));
+
       if (!(active && gtk_widget_get_visible (active)))
         {
           for (i = GTK_MENU_SHELL (priv->popup_widget)->priv->children; i && !active; i = i->next)