test server kill skt with minus 1

test server kill skt with minus 1

The client spams the server with mirror protocol content
and then closes the connection.  However the server wants to
send that content back to all the mirror protocol
connections, including the one that closed.

The result is during the send back phase we usually see an
error trying to send to the close client connection. Because
we don't return -1, we don't close it on server side and the
error repeats a lot before finally closing.  This is a side-
effect of a recent patch to drain pending input before
really closing a dead connection.

This patch just changes it to return -1 on the failed send
attempt assertively closing the connection.

Signed-off-by: Andy Green <andy.green@linaro.org>