fix type error

When selecting an object type from the ``<select>``, there would be an error at line 125 of jquery.fkautocomplete.js:

    TypeError: 'undefined' is not an object (evaluating
    'config.add_urls[config.content_type_id].add_url')

This is because an unset ``<select>`` will give us an undefined ``config.content_type_id``.

Solution: add ``if (config.content_type_id) {`` at line 125, so we don't bother (and we don't need) to do anything in that block until an object type has been selected.