Fix bug lp:782305

Analysis:
Both the wrong result and the valgrind warning were a result
of incomplete cleanup of the MIN/MAX subquery rewrite. At the
first execution of the query, the non-aggregate subquery is
transformed into an aggregate MIN/MAX subquery. During the
fix_fields phase of the MIN/MAX function, it sets the property
st_select_lex::with_sum_func to true.

The second execution of the query finds this flag to be ON.
When optimization reaches the same MIN/MAX subquery
transformation, it tests if the subquery is an aggregate or not.
Since select_lex->with_sum_func == true from the previous
execution, the transformation executes the second branch that
handles aggregate subqueries. This substitutes the subquery
Item into a Item_maxmin_subselect. At the same time elsewhere
it is assumed that the subquery Item is of type
Item_allany_subselect. Ultimately this results in casting the
actual object to the wrong class, and calling the wrong
any_value() method from empty_underlying_subquery().

Solution:
Cleanup the st_select_lex::with_sum_func property in the case
when the MIN/MAX transformation was performed for a non-aggregate
subquery, so that the transformation can be repeated.

sql/item_subselect.h

@@ -366,7 +366,10 @@ TABLE_LIST * const NO_JOIN_NEST=(TABLE_LIST*)0x1;
 #define SUBS_PARTIAL_MATCH_ROWID_MERGE 8
 #define SUBS_PARTIAL_MATCH_TABLE_SCAN 16
 /* ALL/ANY will be transformed with max/min optimization */
-#define SUBS_MAXMIN 32
+/*   The subquery has not aggregates, transform it into a MAX/MIN query. */
+#define SUBS_MAXMIN_INJECTED 32
+/*   The subquery has aggregates, use a special max/min subselect engine. */
+#define SUBS_MAXMIN_ENGINE 64
 
 
 /**
@@ -555,6 +558,7 @@ public:
   Item_allany_subselect(Item * left_expr, chooser_compare_func_creator fc,
                         st_select_lex *select_lex, bool all);
 
+  void cleanup();
   // only ALL subquery has upper not
   subs_type substype() { return all?ALL_SUBS:ANY_SUBS; }
   bool select_transformer(JOIN *join);