Redirect if only one post in category - wordpress

gistfile1.txt

function stf_redirect_to_post(){
   global $wp_query;
   if( is_archive() && $wp_query->post_count == 1 ){
     the_post();
     $post_url = get_permalink();
     // Redirect to post page
     wp_redirect( $post_url );
   }
 } 
 add_action('template_redirect', 'stf_redirect_to_post');