If rendezvous wasn't start or was already stopped ignore Stop call (#1340)

discovery/rendezvous.go

@@ -92,6 +92,9 @@ func (r *Rendezvous) Start() error {
 func (r *Rendezvous) Stop() error {
 	r.mu.Lock()
 	defer r.mu.Unlock()
+	if r.client == nil {
+		return nil
+	}
 	r.cancelRootCtx()
 	if err := r.client.Close(); err != nil {
 		return err

discovery/rendezvous_test.go

@@ -83,6 +83,12 @@ func TestRendezvousRegisterAndDiscoverExitGracefully(t *testing.T) {
 	require.EqualError(t, errDiscoveryIsStopped, err.Error())
 }
 
+func TestStopStoppedNode(t *testing.T) {
+	r, err := NewRendezvous(make([]ma.Multiaddr, 1), nil, nil)
+	require.NoError(t, err)
+	require.NoError(t, r.Stop())
+}
+
 func BenchmarkRendezvousStart(b *testing.B) {
 	identity, err := crypto.GenerateKey()
 	require.NoError(b, err)