Fix bug #14850 Item_ref's null_value wasn't updated

Item_ref's null_value wasn't updated in save_org_in_field() causing reported
error.

mysql-test/r/view.result

@@ -2385,3 +2385,11 @@ show create view v1;
 View	Create View
 v1	CREATE ALGORITHM=UNDEFINED DEFINER=`root`@`localhost` SQL SECURITY INVOKER VIEW `v1` AS select 1 AS `1`
 drop view v1;
+create table t1(f1 int, f2 int);
+insert into t1 values (null, 10), (null,2);
+create view v1 as select * from t1;
+select f1, sum(f2) from v1 group by f1;
+f1	sum(f2)
+NULL	12
+drop view v1;
+drop table t1;

mysql-test/t/view.test

@@ -2253,3 +2253,13 @@ drop view v1;
 create definer = current_user sql security invoker view v1 as select 1;
 show create view v1;
 drop view v1;
+
+#
+# Bug #14850 Item_ref's values wasn't updated
+#
+create table t1(f1 int, f2 int);
+insert into t1 values (null, 10), (null,2);
+create view v1 as select * from t1;
+select f1, sum(f2) from v1 group by f1;
+drop view v1;
+drop table t1;

sql/item.h

@@ -1603,7 +1603,11 @@ public:
   void make_field(Send_field *field);
   bool fix_fields(THD *, Item **);
   int save_in_field(Field *field, bool no_conversions);
-  void save_org_in_field(Field *field)	{ (*ref)->save_org_in_field(field); }
+  void save_org_in_field(Field *field)
+  {
+    (*ref)->save_org_in_field(field);
+    null_value= (*ref)->null_value;
+  }
   enum Item_result result_type () const { return (*ref)->result_type(); }
   enum_field_types field_type() const   { return (*ref)->field_type(); }
   Field *get_tmp_table_field()