_watchdog_get_message_types() is called when watchdog menu isn't expanded

watchdog_link calls _watchdog_get_message_types() even when the watchdog menu isn't expanded. This call causes an expensive and unneccsary SQL query.

This patch sets _watchdog_get_message_types() to be called only when the watchdog menu is expanded.

For details see this thread: http://lists.drupal.org/archives/drupal-devel/2004-02/msg00273.html