hostip.c: return immediately from Curl_resolv_timeout() upon expired timeout.

Ensure existing logic in Curl_resolv_timeout() is not subverted upon getting a
negative timeout from resolve_server(). The timeout in resolve_server() could
be checked to avoid calling Curl_resolv_timeout() with an expired timeout, but
fixing this in this way allows existing logic in resolve_server() to be kept
unchanged.