The problem occurred when one had a subquery that had an equality X=Y where 
Y referred to a named select list expression from the parent select. MySQL 
crashed when trying to use the X=Y equality for ref-based access. 

Fixed by allowing non-Item_field items in the described case.


mysql-test/r/subselect.result:
  BUG#33794 "MySQL crashes executing specific query"
  - Testcase
mysql-test/t/subselect.test:
  BUG#33794 "MySQL crashes executing specific query"
  - Testcase
sql/sql_select.cc:
  BUG#33794 "MySQL crashes executing specific query"
  get_store_key() assumed that if it got a reference
    t.key=Item_outer_ref(Item_direct_ref(x)) 
  then x was an Item_field object, which is not the case when one refers to a
  named select list expression out ot subquery.