INSERT ... SELECT with UNION" (reviewed version).

Altough bug manifest itself only starting from 5.0 it is better to
apply fix to 4.1 to keep some assumptions true and make code more
future-proof.


mysql-test/r/ps.result:
  Added test case for bug #11060 "Server crashes on re-execution of
  prepared INSERT ... SELECT with UNION".
mysql-test/t/ps.test:
  Added test case for bug #11060 "Server crashes on re-execution of
  prepared INSERT ... SELECT with UNION".
sql/sql_insert.cc:
  select_insert::prepare():
    Item::fix_fields() methods operate assuming that LEX::current_select
    points to the select to which current item belongs. Thus
    during check_insert_fields() routine execution LEX::current_select
    should point ot the first select in query since this is the
    select with which items in insert list is associated.
    But if we have INSERT SELECT UNION SELECT type of query
    LEX::current_select will point to the fake_select_lex instead
    since select_insert::prepare() is called during processing of JOIN
    which corresponds to this select_lex.
    So we have set LEX::current_select before calling check_insert_fields()
    and restore it afterwards.