swarm: translate external address candidate

The `NewExternalAddrCandidate` event is yielded both before and after address translation. This will
cause, in the case of TCP, ephemeral ports to be added as candidate.

In turn, that will cause protocols like AutoNAT to fail as these candidates are not actually
reachable/external.

We will now only yield the original candidate if translation did not apply.

Fixes libp2p#4153

swarm/src/lib.rs

@@ -1062,14 +1062,8 @@ where
                 self.pending_event = Some((peer_id, handler, event));
             }
             ToSwarm::NewExternalAddrCandidate(addr) => {
-                self.behaviour
-                    .on_swarm_event(FromSwarm::NewExternalAddrCandidate(
-                        NewExternalAddrCandidate { addr: &addr },
-                    ));
-
-                // Generate more candidates based on address translation.
+                // Apply address translation to the candidate address.
                 // For TCP without port-reuse, the observed address contains an ephemeral port which needs to be replaced by the port of a listen address.
-
                 let translated_addresses = {
                     let mut addrs: Vec<_> = self
                         .listened_addrs
@@ -1083,6 +1077,15 @@ where
                     addrs.dedup();
                     addrs
                 };
+
+                // If address translation yielded nothing, broacast the original candidate address.
+                if translated_addresses.is_empty() {
+                    self.behaviour
+                        .on_swarm_event(FromSwarm::NewExternalAddrCandidate(
+                            NewExternalAddrCandidate { addr: &addr },
+                        ));
+                }
+
                 for addr in translated_addresses {
                     self.behaviour
                         .on_swarm_event(FromSwarm::NewExternalAddrCandidate(