fix bug "Existed user identified by old_password connect failed unless flush privileges"
http://bugs.mysql.com/bug.php?id=68883

Description:
Grant privileges to existed user identified by old_password.
This user connect failed unless flush privileges.

How to repeat:
session 1
mysql> grant all privileges on *.* to 'vin'@localhost identified by 'vinchen1';
Query OK, 0 rows affected (4.83 sec)

mysql> select old_password('vinchen');
+-------------------------+
| old_password('vinchen') |
+-------------------------+
| 7f8da8b51a961f6e        |
+-------------------------+
1 row in set (0.01 sec)

mysql> grant all privileges on *.* to 'vin'@localhost identified by password '7f8da8b51a961f6e';
Query OK, 0 rows affected (2.00 sec)

session 2
shell> mysql -uvin -pvinchen
ERROR 1045 (28000): Access denied for user 'vin'@'localhost' (using password: YES)

session 1
mysql> flush privileges;
Query OK, 0 rows affected (1.37 sec)

session 2
shell> mysql -uvin -pvinchen
--connect success