code: public static Socket safeClose(Socket socket) {
          if (null != socket)
              try {
                  socket.close();
                  socket = null;
              }
              catch (IOException e) {
                  throw Lang.wrapThrow(e);
              }
          return null;
      }
docs: 安全关闭套接层，容忍 null
  
  @param socket
  套接层
  @return 一定会返回 null