Bug #55568: user variable assignments crash server when used

within query

The server could crash after materializing a derived table
which requires a temporary table for grouping.

When destroying the temporary table used to execute a query for
a derived table, JOIN::destroy() did not clean up Item_fields
pointing to fields in the temporary table. This led to
dereferencing a dangling pointer when printing out the items
tree later in the outer SELECT.

The solution is an addendum to the patch for bug37362: in
addition to cleaning up items in tmp_all_fields3, do the same
for items in tmp_all_fields1, since now we have an example
where this is necessary.

sql/sql_select.h

@@ -577,6 +577,7 @@ private:
   */
   bool implicit_grouping; 
   bool make_simple_join(JOIN *join, TABLE *tmp_table);
+  void cleanup_item_list(List<Item> &items) const;
 };