you can try something like:

PHP Code:
//from
$sold_out_button_link zen_href_link(zen_get_info_page($p1['products_id']), 'cPath=' $_GET['cPath'] . '&products_id=' $p1['products_id'], 'SSL');
//to
$sold_out_button_link zen_href_link(zen_get_info_page($p1['products_id']), (isset($_GET['cPath']) ? 'cPath=' $_GET['cPath'] : '') . '&products_id=' $p1['products_id'], 'SSL'); 
best.