MDEV-6743 crash in GROUP_CONCAT(IF () ORDER BY 1)

new fix

sql/sql_select.cc

@@ -21143,7 +21143,7 @@ find_order_in_list(THD *thd, Item **ref_pointer_array, TABLE_LIST *tables,
                order_item->full_name(), thd->where);
       return TRUE;
     }
-    order->item= ref_pointer_array + count - 1;
+    thd->change_item_tree((Item**)&order->item, (Item*)(ref_pointer_array + count - 1));
     order->in_field_list= 1;
     order->counter= count;
     order->counter_used= 1;