multi: prevent nil panics in stop methods.

With this PR we might call the stop method even when the start
method of a subsystem did not successfully finish therefore we
need to make sure we guard the stop methods for potential panics
if some variables are not initialized in the contructors of the
subsystems.

chanfitness/chaneventstore.go

@@ -226,11 +226,17 @@ func (c *ChannelEventStore) Stop() error {
 
 	// Stop the ticker after the goroutine reading from it has exited, to
 	// avoid a race.
-	c.cfg.FlapCountTicker.Stop()
+	var err error
+	if c.cfg.FlapCountTicker == nil {
+		err = fmt.Errorf("ChannelEventStore FlapCountTicker not " +
+			"initialized")
+	} else {
+		c.cfg.FlapCountTicker.Stop()
+	}
 
 	log.Debugf("ChannelEventStore shutdown complete")
 
-	return nil
+	return err
 }
 
 // addChannel checks whether we are already tracking a channel's peer, creates a