update your this function
function start_el(&$output, $item, $depth, $args) {
$url = '#' !== $item->url ? $item->url : '';
$item->title = str_repeat("—", $depth).' '.$item->title;
$output .= '<option value="' . $url . '">' . $item->title.'</option>';
}
and remove or not use function end_el(&$output, $item, $depth)
it will handle your output
<option value="">— Single Post Layouts</option>
<option value="http://domain.com/mag/portfolio/future-semiotics/">—— Full Image Post</option>