Monorail: Fix complete breakage when the user is not signed in.

When the user isn't signed in, there is no star control. Previously, the script wasn't accounting for this and failed with an exception before doing anything useful.
Fixes #12.

MonorailA11yFixes.user.js

@@ -3,9 +3,9 @@
 // @namespace      http://axSGrease.nvaccess.org/
 // @description    Improves the accessibility of Google Code.
 // @author James Teh <jamie@nvaccess.org>
-// @copyright 2016 NV Access Limited
+// @copyright 2016-2017 NV Access Limited
 // @license GNU General Public License version 2.0
-// @version 2016.1
+// @version 2017.1
 // @include https://bugs.chromium.org/p/*/issues/*
 // ==/UserScript==
 
@@ -48,5 +48,10 @@ var observer = new MutationObserver(function(mutations) {
 observer.observe(document, {attributes: true,
 	subtree: true, attributeFilter: ["src"]});
 
-fixStar(document.getElementById("star"));
-makeHeadings();
+function initial() {
+	var star = document.getElementById("star");
+	if (star)
+		fixStar(star);
+	makeHeadings();
+}
+initial();