Pagination negative offset

Imagine page has Pagination,when use follow code to implement Pagination limit

Code:

$data ['list'] = $this->test_model->get_list ( $per_page, $this->uri->segment ( 2 ));

Now lets say user modify segment to -20 (negative value), code will give error

Code:

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '-20, 20' at line 4

LIMIT -20, 20

how we can solved this issue?

currently I'm using following code to avoid mention issue..is any better way?

Code:

$offset = NULL;
if (ctype_digit($this->uri->segment ( 4 ))) {
$offset = $this->uri->segment ( 4 );
}

$data ['list'] = $this->test_model->get_list ( $per_page, $offset);

Thank You