TCPClient crashed when DNS resolving timeout

[zieckey]

Steps:

1. Set the DNS server to a far way server of the local mathine, we set it to 8.8.8.8 in China
2. Set the TCPClient connection timeout to a very small duration, like 1ms

Result:

1. Connector will timeout at first and call Connector::HandleError
2. And then, DNSResolver will timeout later and call Connector::OnDNSResolved with an empty addrs. That will crash.

Test Code:

TEST_UNIT(testTCPClientConnectionTimeout) {
    std::shared_ptr<evpp::EventLoop> loop(new evpp::EventLoop);
    std::shared_ptr<evpp::TCPClient> client(new evpp::TCPClient(loop.get(), "cmake.org:80", "TCPPingPongClient"));
    client->SetConnectionCallback([loop, client](const evpp::TCPConnPtr& conn) {
        loop->Stop();
    });
    client->set_auto_reconnect(false);
    client->set_connecting_timeout(evpp::Duration(0.0001));
    client->Connect();
    loop->Run();
    client.reset();
    loop.reset();
    H_TEST_ASSERT(evpp::GetActiveEventCount() == 0);
}