[crash] fix nil pointer error #749

Signed-off-by: Leo Chen <leo@harmony.one>

p2p/host/hostv2/hostv2.go

@@ -87,6 +87,9 @@ func (r *GroupReceiverImpl) Close() error {
 func (r *GroupReceiverImpl) Receive(ctx context.Context) (
 	msg []byte, sender libp2p_peer.ID, err error,
 ) {
+	if r.sub == nil {
+		return nil, libp2p_peer.ID(""), fmt.Errorf("Receive: r.sub == nil")
+	}
 	m, err := r.sub.Next(ctx)
 	if err == nil {
 		msg = m.Data