slirp: Handle error returns from slirp_send() in sosendoob()

The code in sosendoob() assumes that slirp_send() always
succeeds, but it might return an OS error code (for instance
if the other end has disconnected). Catch these and return
the caller either -1 on error or the number of urgent bytes
actually written. (None of the callers check this return
value currently, though.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
