Fix exceptions in rump connections.

[akuktin]

In the Wild West that is the public Internet, you sometimes get a connection that executes all or a part of the TLS handshake and then does nothing or drops the connection. sstp-server starts a Hello timer as soon as a connection is established, and it will expire some time after the client drops the connection. Since the SSTPProtocol object doesn't have the transport attribute when the Hello timer expires, an exception is generated.

BTW, a review of code should be performed since it's possible there's a memory leak in these or other pathologic situations.