Description of problem:
UNSIGNED update error

Version-Release number of selected component (if applicable):
5.1.52

How reproducible:
[root@wifi dobs]# mysql --password=***
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Your MySQL connection id is 23
Server version: 5.1.52 Source distribution

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mysql> CREATE TABLE `demo`.`test` (
    -> `id` INT( 3 ) NOT NULL AUTO_INCREMENT PRIMARY KEY ,
    -> `int1` INT( 4 ) UNSIGNED NOT NULL ,
    -> `int2` INT( 4 ) UNSIGNED NOT NULL 
    -> ) ENGINE = MYISAM ;
Query OK, 0 rows affected (0.11 sec)

mysql> INSERT INTO `demo`.`test` (`id` ,`int1` ,`int2` ) VALUES (NULL , '0', '10');
Query OK, 1 row affected (0.00 sec)

mysql> SELECT * FROM `demo`.`test`;
+----+------+------+
| id | int1 | int2 |
+----+------+------+
|  1 |    0 |   10 |
+----+------+------+
1 row in set (0.00 sec)

mysql> UPDATE `demo`.`test` SET `int2` = '9' WHERE `test`.`id` =1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> SELECT * FROM `demo`.`test`;
+----+------+------+
| id | int1 | int2 |
+----+------+------+
|  1 |    0 |    9 |
+----+------+------+
1 row in set (0.00 sec)

mysql> UPDATE `demo`.`test` SET `int1` = (`int1`-1), `int2` = (`int2`-1) WHERE
`test`.`id` =1;
Query OK, 1 row affected, 1 warning (0.04 sec)
Rows matched: 1  Changed: 1  Warnings: 1

mysql> SELECT * FROM `demo`.`test`;
+----+------------+------+
| id | int1       | int2 |
+----+------------+------+
|  1 | 4294967295 |    8 |
+----+------------+------+
1 row in set (0.00 sec)

  
Actual results:
|  1 | 4294967295 |    8 |


Expected results:
|  1 | 0 |    8 |

Additional info:
+ open bug on mysql http://bugs.mysql.com/bug.php?id=59426