p2p/nat: improve AddMapping code (ethereum#31486)

It introduces a new variable to store the external port returned by the
addAnyPortMapping function and ensures that the correct external port is
returned even in case of an error.

---------

Co-authored-by: Felix Lange <fjl@twurst.com>

Author: gazzua

p2p/nat/natupnp.go

@@ -82,7 +82,7 @@ func (n *upnp) ExternalIP() (addr net.IP, err error) {
 func (n *upnp) AddMapping(protocol string, extport, intport int, desc string, lifetime time.Duration) (uint16, error) {
 	ip, err := n.internalAddress()
 	if err != nil {
-		return 0, nil // TODO: Shouldn't we return the error?
+		return 0, err
 	}
 	protocol = strings.ToUpper(protocol)
 	lifetimeS := uint32(lifetime / time.Second)
@@ -94,14 +94,15 @@ func (n *upnp) AddMapping(protocol string, extport, intport int, desc string, li
 	if err == nil {
 		return uint16(extport), nil
 	}
-
-	return uint16(extport), n.withRateLimit(func() error {
+	// Try addAnyPortMapping if mapping specified port didn't work.
+	err = n.withRateLimit(func() error {
 		p, err := n.addAnyPortMapping(protocol, extport, intport, ip, desc, lifetimeS)
 		if err == nil {
 			extport = int(p)
 		}
 		return err
 	})
+	return uint16(extport), err
 }
 
 func (n *upnp) addAnyPortMapping(protocol string, extport, intport int, ip net.IP, desc string, lifetimeS uint32) (uint16, error) {