NETINFO clock skew detection doesn't work on clients

The NETINFO clock skew detection code in channel_tls_process_netinfo_cell() doesn't work for clients because of an apparently unnecessary call to router_get_by_id_digest(), which will generally return NULL on clients (which generally download only microdescriptors therefore won't populate the routerlist). This NULL return prevented the clock skew reporting code from running. Thanks to arma for pointing this out on IRC. Fixing this would help bootstrap fail more quickly in some clock skew situations.