xio_connection.c/xio_send_response: fix use-after-free

[redlicha]

Prevent a use-after-free in xio_send_response when discarding messages in case the client code frees up the xio_msg in its on_msg_error callback. Another potential use-after-free is prevented by not using connection in that case anymore, as the client could have invoked xio_connection_destroy as well.

Addresses #8