In UNION if we use last SELECT without braces and this
SELECT have ORDER BY clause, such clause belongs to
global UNION. It is parsed like last SELECT
part and used further as 'unit->global_parameters->order_list' value.
During DESCRIBE EXTENDED we call select_lex->print_order() for
last SELECT where order fields refer to tmp table 
which already freed. It leads to crash.
The fix is clean up global_parameters->order_list
instead of fake_select_lex->order_list.


mysql-test/r/union.result:
  test result
mysql-test/t/union.test:
  test case
sql/sql_union.cc:
  In UNION if we use last SELECT without braces and this
  SELECT have ORDER BY clause, such clause belongs to
  global UNION. It is parsed like last SELECT
  part and used further as 'unit->global_parameters->order_list' value.
  During DESCRIBE EXTENDED we call select_lex->print_order() for
  last SELECT where order fields refer to tmp table 
  which already freed. It leads to crash.
  The fix is clean up global_parameters->order_list
  instead of fake_select_lex->order_list.