slirp_update_timeout: Fix handling of *timeout = 0
libslirp-rs in https://gitlab.freedesktop.org/slirp/libslirp-rs/-/blob/a82279f34a6896ff77e05e0af4828c2d5c900f34/src/mio.rs#L286-287
*timeout = 0 before calling
Currently, this results in
*timeout staying at 0 and all (TCP) timeouts never firing on their own
(they will only fire randomly if a file descriptor becomes ready eventually).
You could, alternatively, argue that libslirp-rs should
let mut timeout = u32::MAX but 0 seems like
a more reasonable default value for timeout.
Signed-off-by: Rafael Gieschke email@example.com