results." a.k.a. "Proper cleanup of subqueries is missing for SET and DO
statements". (Version #2 with after-review fixes).

To perform proper cleanup for statements that can contain subqueries 
but don't have main select we must call free_undelaid_joins().


mysql-test/r/subselect.result:
  Added test for bug #6462 "Same request on same data returns different
  results." a.k.a. "Proper cleanup of subqueries is missing for SET and DO
  statements".
mysql-test/t/subselect.test:
  Added test for bug #6462 "Same request on same data returns different
  results." a.k.a. "Proper cleanup of subqueries is missing for SET and DO
  statements".
sql/set_var.cc:
  Added missing cleanup of joins used in subqueries to SET statement.
sql/sql_do.cc:
  Added missing cleanup of joins used in subqueries to DO statement.