Fix bug #15706 find_field_in_tables() returns field from outer select

If item->cached_table is set, find_field_in_tables() returns found field
even if it doesn't belong to current select. Because Item_field::fix_fields
doesn't expect such behaviour, reported bug occurs.

Item_field::fix_fields() was modifed to detect when find_field_in_tables() 
can return field from outer select and process such fields accordingly.
In order to ease this code which was searching and processing outed fields was
moved into separate function called Item_field::fix_outer_field().

mysql-test/t/disabled.def

@@ -12,4 +12,3 @@
 
 sp-goto         : GOTO is currently is disabled - will be fixed in the future
 kill            : Unstable test case, bug#9712
-subselect       : Bug#15706

sql/item.h

@@ -1161,6 +1161,7 @@ class Item_field :public Item_ident
   inline uint32 max_disp_length() { return field->max_length(); }
   Item_field *filed_for_view_update() { return this; }
   Item *safe_charset_converter(CHARSET_INFO *tocs);
+  int fix_outer_field(THD *thd, Field **field, Item **reference);
   friend class Item_default_value;
   friend class Item_insert_value;
   friend class st_select_lex_unit;

sql/item_subselect.h

@@ -125,6 +125,7 @@ class Item_subselect :public Item_result_field
   friend class select_subselect;
   friend class Item_in_optimizer;
   friend bool Item_field::fix_fields(THD *, Item **);
+  friend int  Item_field::fix_outer_field(THD *, Field **, Item **);
   friend bool Item_ref::fix_fields(THD *, Item **);
   friend void mark_select_range_as_dependent(THD*,
                                              st_select_lex*, st_select_lex*,