Don't call member functions for a NIL pointer.


mysql-test/r/subselect4.result:
  Add test case.
mysql-test/t/subselect4.test:
  Add test case.
sql/sql_select.cc:
  If the (virtual) member function clone_item() returns NULL,
  there is no substitution to be made, and we don't need to set the collation.
  The test was invoking Item_cache::clone_item()