I'm trying to redirect users from an existing path in Drupal 7 to another existing path. I want to implement the following rule:

comment/reply/123 should redirect to node/123#comment-form

But

comment/reply/123/456 should NOT redirect anywhere and just go where it goes.

I've tried to implement this behavior with hook_menu(), but here's the problem. When I register an item with the path 'comment/reply/%' that corresponds to comment/reply/123, it automatically includes paths like comment/reply/123/456. I know that's intended and changed my code to include this case, but can't figure out how to redirect users only when they hit comment/reply/123. Here's my code:

function mymodule_menu()
{
    $items = array();
    $items['comment/reply/%/%'] = array(
        'title' => 'bla',
        'page callback' => '_mymodule_redirect',
        'page arguments' => array(2, 3),
        'access callback' => TRUE,
        'type' => MENU_CALLBACK,
    );
    return $items;
}

function _mymodule_redirect($node_id, $comment_id)
{
    if(isset($comment_id))
    {
        return; //I want to just go to comment/reply/123/456 here, but it gives me a blank page
    }
    else
    {
        drupal_goto('node/' . $node_id . '#comment-form');
    }
}

So how do I go where I want to go?

Update: This is my final solution: I now use hook_menu_alter() to rewrite an already defined link and comment_reply() to serve standard comment form.

function mymodule_menu_alter(&$items) 
{
  $items['comment/reply/%node']['page callback'] = '_mymodule_redirect';
}

function _mymodule_redirect($node, $pid = NULL)
{
    if(isset($pid))
    {
         return comment_reply($node, $pid);
    }
    else
    {
        drupal_goto(drupal_get_path_alias('node/' . $node->nid), array('fragment' => 'comment-form'), 301);
    }
}