fix cannot use backup driver

$ci = &get_instance();
$ci->load->driver('cache', array('adapter' => 'memcached', 'backup' => 'file'));

$ci->cache->get('cache_key');  // if the Memcached(Memcache) Extension does not exist, it will not work.
// Fatal error:  Call to a member function get() on a non-object in .../system/libraries/Cache/drivers/Cache_memcached.php on line 50