This bug was introduced in this revision: 
kostja@sun.com-20100727102553-b4n2ojcyfj79l2x7
("A pre-requisite patch for the fix for Bug#52044.")

It happens because close_thread_tables() is now called in
open_and_lock_tables upon failure.  Hence, table is no longer
open when optimizer tries to do cleanup.

Fix: Make sure to do cleanup in st_select_lex_unit::prepare()
upon failure. This way, cleanup() is called before tables are
released.

mysql-test/r/subselect.result:
  Added test case for Bug#57704.
mysql-test/t/subselect.test:
  Added test case for Bug#57704.
sql/sql_union.cc:
  st_select_lex_unit::prepare():  On failure, make sure cleanup()
  is called.