UDS: prevent crash when proxy udp (#3967)

* net: Prevent nil pointer err in NetAddr()

* Fix dsworker saddr problem

app/proxyman/inbound/worker.go

@@ -464,7 +464,8 @@ func (w *dsWorker) callback(conn stat.Connection) {
 		}
 	}
 	ctx = session.ContextWithInbound(ctx, &session.Inbound{
-		Source:  net.DestinationFromAddr(conn.RemoteAddr()),
+		// Unix have no source addr, so we use gateway as source for log.
+		Source:  net.UnixDestination(w.address),
 		Gateway: net.UnixDestination(w.address),
 		Tag:     w.tag,
 		Conn:    conn,

common/net/destination.go

@@ -89,11 +89,13 @@ func UnixDestination(address Address) Destination {
 // NetAddr returns the network address in this Destination in string form.
 func (d Destination) NetAddr() string {
 	addr := ""
+	if d.Address != nil {
 		if d.Network == Network_TCP || d.Network == Network_UDP {
 			addr = d.Address.String() + ":" + d.Port.String()
 		} else if d.Network == Network_UNIX {
 			addr = d.Address.String()
 		}
+	}
 	return addr
 }