fix crash in notifications

src/notifications.js

@@ -444,7 +444,7 @@ var async = require('async'),
 					case 'notifications:user_posted_to':
 					case 'notifications:user_flagged_post_in':
 						var usernames = set.map(function(notifObj) {
-							return notifObj.user.username;
+							return notifObj && notifObj.user && notifObj.user.username;
 						}).filter(function(username, idx, array) {
 							return array.indexOf(username) === idx;
 						});