trac 18 deal with service connection timing out

While looking at http://libwebsockets.org/trac/ticket/18
noticed the flow for timeout in service_fd will do bad things
if the fd we came to service has timed out.  It gets freed and
then "serviced'.

Reported-by: Joakim Soderberg <joakim.soderberg@gmail.com>
Signed-off-by: Andy Green <andy.green@linaro.org>