removed annecessary assignment found by Monty

sql/item_subselect.cc

@@ -152,9 +152,7 @@ String *Item_singleval_subselect::val_str (String *str)
     assign_null();
     return 0;
   }
-  // Assign temporary buffer with stored value
-  str_value.set(string_value, 0, string_value.length());
-  return &str_value;
+  return &string_value;
 }
 
 Item_exists_subselect::Item_exists_subselect(THD *thd,