message: Forbid spec usage of "slaveof" command
  Forbid this case
  A is master of B, A want to slaveof C
diff: @@ -43,6 +43,12 @@ void SlaveofCmd::DoInitial() {
  return;
  }
  + // self is master of A , want to slavof B
  + if (g_pika_server->role() & PIKA_ROLE_MASTER) {
  + res_.SetRes(CmdRes::kErrOther, "already master of others, invalid usage");
  + return;
  + }
  +
  master_ip_ = argv_[1];
  std::string str_master_port = argv_[2];
  if (!slash::string2l(str_master_port.data(), str_master_port.size(), &master_port_) || master_port_ <= 0) {
  