Resolving a racing condition while adding ports on the app's side.

An earlier attempt (ad6265786871) to resolve this condition on the
router's side added a new issue: the app could get a request before
acquiring a port.

go/port.go

@@ -50,7 +50,11 @@ func add_port(p *port) {
 		port_registry_.m = make(map[port_key]*port)
 	}
 
-	port_registry_.m[p.key] = p
+	old := port_registry_.m[p.key]
+
+	if old == nil {
+		port_registry_.m[p.key] = p
+	}
 
 	port_registry_.Unlock()
 }