Fix panic when DNS resolution for STUN server returns only IPv6 addrs. (#402)

easytier/src/common/stun.rs

@@ -56,6 +56,8 @@ impl HostResolverIter {
                     self.ips = ips
                         .filter(|x| x.is_ipv4())
                         .choose_multiple(&mut rand::thread_rng(), self.max_ip_per_domain as usize);
+                    
+                    if self.ips.is_empty() {return self.next().await;}
                 }
                 Err(e) => {
                     tracing::warn!(?host, ?e, "lookup host for stun failed");