slirp: avoid use-after-free in slirp_pollfds_poll() if soread() returns an error

Samuel Thibault pointed out that it's possible that slirp_pollfds_poll()
will try to use a socket even after soread() returns an error, resulting
in an use-after-free if the socket was removed while handling the error.
Avoid this by refusing to continue to work with the socket in this case.

Signed-off-by: Steven Luo <steven+qemu@steven676.net>
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
