Make *id fields unsigned.
function dblog_update_8001() {
db_drop_index('watchdog', 'uid');
db_change_field('watchdog', 'uid', 'uid', array(
'type' => 'int',
'unsigned' => TRUE,
'not null' => TRUE,
'default' => 0,
'description' => 'The {users}.uid of the user who triggered the event.',
), array(
'indexes' => array(
'uid' => array(
'uid',
),
),
));
}