Transporter::doSend() Fails to update send buffers after a partly failed send

If some data were sent (sum_sent > 0) before my_socket_writev
returned with an error, we returned immediately without
calling iovec_data_sent() which maintains the send buffers.

Thus another ::doSend() will fetch some of the already sent 
data. Iff that send does not fail, the prev sent data is 
actually sent twice - Resulting in garbage received by the
destination node.