init values to avoid junk returning in case of null value asking without assigning value (BUG#6806)

mysql-test/r/subselect.result:
   Equal operation under row and empty subquery
mysql-test/t/subselect.test:
   Equal operation under row and empty subquery
sql/item.h:
  init values to avoid junk returning in case of null value asking without assigning value

mysql-test/r/subselect.result

@@ -2005,3 +2005,8 @@ explain select a from t1 where c=2;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	3	Using where
 drop table t1;
+create table t1 (s1 int,s2 int);
+insert into t1 values (20,15);
+select * from t1 where  (('a',null) <=> (select 'a',s2 from t1 where s1 = 0));
+s1	s2
+drop table t1;

mysql-test/t/subselect.test

@@ -1301,3 +1301,11 @@ explain select a from t1 where c=2;
 do @a:= (select sum(a) from t1 where b > @b);
 explain select a from t1 where c=2;
 drop table t1;
+
+#
+# Equal operation under row and empty subquery
+#
+create table t1 (s1 int,s2 int);
+insert into t1 values (20,15);
+select * from t1 where  (('a',null) <=> (select 'a',s2 from t1 where s1 = 0));
+drop table t1;

sql/item.h

@@ -1127,7 +1127,7 @@ class Item_cache_int: public Item_cache
 {
   longlong value;
 public:
-  Item_cache_int(): Item_cache() {}
+  Item_cache_int(): Item_cache(), value(0) {}
   
   void store(Item *item);
   double val() { DBUG_ASSERT(fixed == 1); return (double) value; }
@@ -1145,7 +1145,7 @@ class Item_cache_real: public Item_cache
 {
   double value;
 public:
-  Item_cache_real(): Item_cache() {}
+  Item_cache_real(): Item_cache(), value(0) {}
 
   void store(Item *item);
   double val() { DBUG_ASSERT(fixed == 1); return value; }
@@ -1167,7 +1167,7 @@ class Item_cache_str: public Item_cache
   char buffer[80];
   String *value, value_buff;
 public:
-  Item_cache_str(): Item_cache() { }
+  Item_cache_str(): Item_cache(), value(0) { }
   
   void store(Item *item);
   double val();