[#1291] innerring: Don't stop the same listener twice

When IR node configured without main chain, both
`morphListener` and `mainnetListener` are pointing
into single listener component. We should not call
`Stop()` twice, because it may trigger channel
closing in neo-go or other components and it
can throw panic.

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

pkg/innerring/innerring.go

@@ -282,7 +282,9 @@ func (s *Server) Stop() {
 	s.setHealthStatus(control.HealthStatus_SHUTTING_DOWN)
 
 	go s.morphListener.Stop()
-	go s.mainnetListener.Stop()
+	if !s.withoutMainNet {
+		go s.mainnetListener.Stop()
+	}
 
 	for _, c := range s.closers {
 		if err := c(); err != nil {