Patch for Drupal.dhtmlMenu.cookieSet() capability with latest Jquery versions

I've update Jquery to 1.7 version (#1067290: Fix jQuery 1.7 for Drupal 6) and faced a little problem in Drupal.dhtmlMenu.cookieSet() function.

Drupal.dhtmlMenu.cookieSet = function() {
  var expanded = new Array();
  $('li.expanded').each(function() {
    expanded.push($(this).find('a:first').attr('id').substr(5));
  });
  document.cookie = 'dhtml_menu=' + expanded.join(',') + ';path=/';
}

In 1.3 version of Jquery .attr('id') returned empty string if id wasn't defined. But in recent versions it returns undefined.
So we should check the return before call .substr() method.