The report_session_error() fix does not work all the times. Proper fix here.(Merged from MOODLE_15_STABLE)

lib/moodlelib.php

@@ -6528,7 +6528,7 @@ function remove_dir($dir, $content_only=false) {
 }
 
 function report_session_error() {
-    global $CFG, $FULLME;
+    global $CFG, $FULLME, $SESSOIN;
     if (empty($CFG->lang)) {
         $CFG->lang = "en";
     }
@@ -6544,6 +6544,7 @@ function report_session_error() {
     }
     unset($_SESSION['USER']);
     unset($GLOBALS['USER']);
+    unset($SESSION->session_test);
     redirect($FULLME, get_string('sessionerroruser', 'error'), 2);
 }