Blocked evaluation of constant objects of the classes
Item_func_is_null and Item_is_not_null_test at the
prepare phase in the cases when the objects used subqueries. 


mysql-test/r/ps.result:
  Extended test case for bug #25027.
mysql-test/t/ps.test:
  Extended test case for bug #25027.
sql/sql_lex.cc:
  Returned back the assertion in st_select_lex_unit::set_limit,
  removed by the previous commit for this bug.