Using more Item_string_sys

sql/set_var.h

@@ -238,9 +238,7 @@ public:
     if (value_arg && value_arg->type() == Item::FIELD_ITEM)
     {
       Item_field *item= (Item_field*) value_arg;
-      if (!(value=new Item_string(item->field_name,
-                                  (uint) strlen(item->field_name),
-                                  system_charset_info))) // names are utf8
+      if (!(value=new Item_string_sys(item->field_name))) // names are utf8
         value=value_arg;                        /* Give error message later */
     }
     else