sql/sql_derived.cc:
  memory leack fixed
sql/sql_lex.cc:
  flag to avoid double cleaning
sql/sql_lex.h:
  flag to avoid double cleaning
sql/sql_select.cc:
  right way to cleanup JOIN_TAB
sql/sql_select.h:
  right way to cleanup JOIN_TAB
sql/sql_union.cc:
  flag to avoid double cleaning