删除公告，仍有未读的问题

application/api/controller/Message.php

@@ -23,7 +23,7 @@ class Message extends Api
         //系统消息未读数量
         $all_num = Db::name('message_sys')->count();
         $read_num = Db::name('user_messagesys')->where('user_id',$this->auth->id)->count();
-        $rs['msgsys_unread_num'] = abs($all_num - $read_num);
+        $rs['msgsys_unread_num'] = $all_num > $read_num ? $all_num - $read_num : 0; //因公告被删除，未读公告没来得及删除，相减可能会负数
 
         $this->success(1,$rs);
     }