mysql-test/r/subselect.result:
  fixed drop table statement
  test for BUG#1536
mysql-test/t/subselect.test:
  fixed drop table statement
  test for BUG#1536
sql/sql_derived.cc:
  all tree of derived table should be deleted
sql/sql_lex.cc:
  delete subtree method
sql/sql_lex.h:
  delete subtree method