Account for hidden terms when checking if node preview is styled

In function advanced_forum_is_forum_tagged, in includes/style.inc, there is this code:

 if ($preview) {
    foreach ($node->taxonomy_forums[$langcode] as $tforum) {
      if ($tforum['taxonomy_term']->vid == $vid) {
        return TRUE;
      }
    }

When the term field is hidden in the default display mode, it bypasses the code that adds the taxonomy_term element and all that is there is the tid. This code should check for that otherwise it throws an undefined index error when previewing a new forum node.