Added a workaround to prevent the plugin from triggering notices when it tries to display a link in a comment that belongs to a post of a CPT that has since been removed.  In other words, dealing with invalid CPTs.

git-svn-id: https://plugins.svn.wordpress.org/broken-link-checker/trunk@1024971 b8457f37-d9ea-0310-8a92-e5e31aec5664