Don't let unreadCount become negative

If you had an inaccurate unread count due to previously broken unread
tracking, it's possible to go negative and never fully recover. Fixed
by clamping to zero.

// FREEBIE

js/conversation_controller.js

@@ -30,6 +30,7 @@
         updateUnreadCount: function(model, count) {
             var prev = model.previous('unreadCount') || 0;
             var newUnreadCount = storage.get("unreadCount", 0) - (prev - count);
+            if (newUnreadCount < 0) { newUnreadCount = 0; }
             storage.remove("unreadCount");
             storage.put("unreadCount", newUnreadCount);