If elements a not top-level IN subquery were accessed by an index and 
the subquery result set included  a NULL value then the quantified
predicate that contained the subquery was evaluated to NULL when 
it should return a non-null value.


mysql-test/r/subselect.result:
  Added a test case for bug #23478.
mysql-test/t/subselect.test:
  Added a test case for bug #23478.