MDL-62889 message_popup: redirect to notification page if url is empty

If you pass a URL that is not a valid URL (for example
';') it is cleaned to an empty string which redirects to
$CFG->wwwroot/message/output/popup/ which is not a valid
page.

message/output/popup/mark_notification_read.php

@@ -31,10 +31,15 @@
 }
 
 $notificationid = required_param('notificationid', PARAM_INT);
-$redirecturl = optional_param('redirecturl', $CFG->wwwroot, PARAM_URL);
+$redirecturl = optional_param('redirecturl', '', PARAM_URL);
 
 $notification = $DB->get_record('message', array('id' => $notificationid, 'notification' => 1));
 
+// If the redirect URL after filtering is empty, or it was never passed, then redirect to the notification page.
+if (empty($redirecturl)) {
+    $redirecturl = new moodle_url('/message/output/popup/notifications.php', ['notificationid' => $notificationid]);
+}
+
 // If found, is unread, so mark read if belongs to this user.
 if ($notification) {
     if ($USER->id == $notification->useridto) {