Don't go into branch that calls upper_refs.empty() more than once per
  PREPARE or EXECUTE
* Avoid crashing when processing references to outside from subquery's HAVING
  (will explain in more details in email)

sql/item.h:
  * Avoid crashing when processing references to outside from subquery's HAVING
    (will explain in more details in email)
sql/item_subselect.cc:
  * Better self-recursion protection in Item_subselect::fix_fields. 
    Don't go into branch that calls upper_refs.empty() more than once per
    PREPARE or EXECUTE