Fix a potential conflict with plugins that use PHP sessions.

PHP sessions are blocking. session_start() will wait until all other scripts that are using the same session are finished. As a result, a long-running script that unintentionally keeps the session open can cause the entire site to "lock up" for the current user/browser. BLC itself does not start sessions, but some plugins start a session on every pageload, which includes the BLC worker script triggered via AJAX/Cron. So sometimes the WP admin may become unresponsive while the worker is running.

Fixed by making the worker close the current session (if one exists) before doing anything else.

git-svn-id: https://plugins.svn.wordpress.org/broken-link-checker/trunk@1012290 b8457f37-d9ea-0310-8a92-e5e31aec5664