Fix for valgrind warning introduced by the fix for bug#21354:

(COUNT(*) = 1) not working in SELECT inside prepared statement.
Note: the warning was introduced in 5.0 and 5.1, 4.1 is OK with the
original fix.

The problem was that in 5.0 and 5.1 clear() for group functions may
access hybrid_type member, and this member is initialized in
fix_fields().

So we should not call clear() from item cleanup() methods, as cleanup()
may be called for unfixed items.

sql/item_sum.cc

@@ -1055,7 +1055,7 @@ longlong Item_sum_count::val_int()
 void Item_sum_count::cleanup()
 {
   DBUG_ENTER("Item_sum_count::cleanup");
-  clear();
+  count= 0;
   Item_sum_int::cleanup();
   used_table_cache= ~(table_map) 0;
   DBUG_VOID_RETURN;

sql/item_sum.h

@@ -643,8 +643,8 @@ class Item_sum_avg :public Item_sum_sum
   Field *create_tmp_field(bool group, TABLE *table, uint convert_blob_length);
   void cleanup()
   {
-    clear();
-    Item_sum_num::cleanup();
+    count= 0;
+    Item_sum_sum::cleanup();
   }
 };
 
@@ -727,7 +727,8 @@ class Item_sum_variance : public Item_sum_num
   enum Item_result result_type () const { return REAL_RESULT; }
   void cleanup()
   {
-    clear();
+    cur_dec= 0;
+    count= 0;
     Item_sum_num::cleanup();
   }
 };
@@ -862,7 +863,7 @@ class Item_sum_bit :public Item_sum_int
   { decimals= 0; max_length=21; unsigned_flag= 1; maybe_null= null_value= 0; }
   void cleanup()
   {
-    clear();
+    bits= reset_bits;
     Item_sum_int::cleanup();
   }
 };