Added a partial workaround for situations where a custom post type is removed without removing the actual posts that use that CPT. The custom field container still doesn't handle that situation very well, but at least it won't throw PHP notices needlessly.

git-svn-id: https://plugins.svn.wordpress.org/broken-link-checker/trunk@1020129 b8457f37-d9ea-0310-8a92-e5e31aec5664