Removing example.com links prevents linkchecker from working

If you set the textarea for 'Do not check the link status of links containing these URLs' blank, linkchecker sets the status of everything in linkchecker_links table to 0 and nothing gets checked.

It took me a while to track this down and get linkchecker working. There must be a problem with the preg_match logic in _linkchecker_link_check_status_filter. The easiest fix may be to make that setting a required field.