using a derived table over a grouping subselect.

This crash happens only when materialization of the derived tables 
requires creation of auxiliary temporary table, for example when
a grouping operation is carried out with usage of a temporary table.

The crash happened because EXPLAIN EXTENDED when printing the query
expression made an attempt to use the objects created in the mem_root
of the temporary table which has been already freed by the moment
when printing is called.

This bug appeared after the method Item_field::print() had been 
introduced.