nid not properly set on comment form on non-node pages

I have a use case where I'm adding a comment to a node but not on a node page. When the comment is submitted everything works as it should, except that the comment isn't associated with an nid. So on page reload it's no longer there.

I tracked the problem down to the comment_form_alter in the module:

function ajax_comments_form_comment_form_alter(&$form, $form_state) {
  $node = menu_get_object();

  if (!isset($node) && arg(0) == 'ajax_comments') {
    $node = node_load(arg(2));
  }

In the case where the ajax comments is on a node page, then menu_get_object() should work. But when it's not then $node won't be set at all. Since the comment form already has an nid value this should be used as a fallback.

I've attached a small patch that should fix the issue.