Item::str_value can't be used for storing value

(fixed error in Item_cache_str)

sql/item.cc

@@ -1630,11 +1630,11 @@ Item_cache* Item_cache::get_cache(Item_result type)
 
 void Item_cache_str::store(Item *item)
 {
-  str_value.set(buffer, sizeof(buffer), item->charset());
-  value= item->str_result(&str_value);
+  value_buff.set(buffer, sizeof(buffer), item->charset());
+  value= item->str_result(&value_buff);
   if ((null_value= item->null_value))
     value= 0;
-  else if (value != &str_value)
+  else if (value != &value_buff)
   {
     /*
       We copy string value to avoid changing value if 'item' is table field
@@ -1644,10 +1644,10 @@ void Item_cache_str::store(Item *item)
              (select c from t1 where a=t2.a)
         from t2;
     */
-    str_value.copy(*value);
-    value= &str_value;
+    value_buff.copy(*value);
+    value= &value_buff;
   }
-
+  set_charset(&item->collation);
 }
 double Item_cache_str::val()
 { 

sql/item.h

@@ -920,7 +920,7 @@ class Item_cache_real: public Item_cache
 class Item_cache_str: public Item_cache
 {
   char buffer[80];
-  String *value;
+  String *value, value_buff;
 public:
   Item_cache_str(): Item_cache() { }