TcpResolver: reduce risk of accessing destructed TcpConnection
TcpResolver: reduce risk of accessing destructed TcpConnection

Invoking TcpHandler::onError might result in the connection being
destroyed.  Though the reference to it in TcpClosed() is likely benign,
it's safer to follow the standard practice of returning a nullptr to
indicate that the connection is known to be destroyed.

toolking/AMQP-CPP,CopernicaMarketingSoftware/AMQP-CPP,toolking/AMQP-CPP,CopernicaMarketingSoftware/AMQP-CPP