fix of selfreference bug

mysql-test/r/subselect.result

@@ -15,6 +15,8 @@ Reference 'b' not supported (forward reference in item list)
 SELECT (SELECT 1),MAX(1) FROM (SELECT 1);
 (SELECT 1)	MAX(1)
 1	1
+SELECT (SELECT a) as a;
+Reference 'a' not supported (forward reference in item list)
 drop table if exists t1,t2,t3,t4,t5,attend,clinic,inscrit;
 create table t1 (a int);
 create table t2 (a int, b int);

mysql-test/t/subselect.test

@@ -6,6 +6,8 @@ SELECT (SELECT 1 FROM (SELECT 1) HAVING a=1) as a;
 -- error 1245
 SELECT (SELECT 1 FROM (SELECT 1) HAVING b=1) as a,(SELECT 1 FROM (SELECT 1) HAVING a=1) as b;
 SELECT (SELECT 1),MAX(1) FROM (SELECT 1);
+-- error 1245
+SELECT (SELECT a) as a;
 drop table if exists t1,t2,t3,t4,t5,attend,clinic,inscrit;
 create table t1 (a int);
 create table t2 (a int, b int);

sql/item.h

@@ -403,8 +403,8 @@ public:
     :Item_ident(NullS,table_name_par,field_name_par),ref(item) {}
   enum Type type() const		{ return REF_ITEM; }
   bool eq(const Item *item, bool binary_cmp) const
-  { return (*ref)->eq(item, binary_cmp); }
-  ~Item_ref() { if (ref) delete *ref; }
+  { return ref && (*ref)->eq(item, binary_cmp); }
+  ~Item_ref() { if (ref && (*ref) != this) delete *ref; }
   double val()
   {
     double tmp=(*ref)->val_result();