fix(connector/relay): handle empty address in bind to fix UDP ASSOCIATE

Fixes a bug where an empty bind address (as in SOCKS5 UDP ASSOCIATE) bypassed the SplitHostPort guard and passed `""` to `AddrFeature.ParseFrom`, which returns an error for empty hosts. Default to `0.0.0.0:0` (any address, any port) when address is empty.

connector/relay/bind.go

@@ -106,7 +106,9 @@ func (c *relayConnector) bind(conn net.Conn, cmd relay.CmdType, network, address
 	})
 	fa := &relay.AddrFeature{}
 	if network != "unix" {
-		if h, _, e := net.SplitHostPort(address); e == nil && h == "" {
+		if address == "" {
+	        address = "0.0.0.0:0"
+	    } else if h, _, e := net.SplitHostPort(address); e == nil && h == "" {
 	        address = net.JoinHostPort("0.0.0.0", address)
 	    }
 	}