Merge pull request 'Check if Listener exists before trying to close it' (#26) from bugfix into master

Reviewed-on: https://git.openprivacy.ca/cwtch.im/tapir/pulls/26

networks/tor/BaseOnionService.go

@@ -172,7 +172,9 @@ func (s *BaseOnionService) Listen(app tapir.Application) error {
 func (s *BaseOnionService) Shutdown() {
 	s.lock.Lock()
 	defer s.lock.Unlock()
-	s.ls.Close()
+	if s.ls != nil {
+		s.ls.Close()
+	}
 	s.connections.Range(func(key, value interface{}) bool {
 		connection := value.(tapir.Connection)
 		connection.Close()