Fixed a small bug that prevented guest users from log in

See: http://moodle.org/bugs/bug.php?op=show&bugid=3989
Thx to [email protected]

auth/shibboleth/index.php

@@ -4,7 +4,7 @@
     require('../../config.php');
     require('lib.php');
 
-    if (isloggedin()) {      // Nothing to do
+    if (isloggedin() && $USER->username != 'guest') {      // Nothing to do
         redirect($CFG->wwwroot.'/index.php');
     }