eth/txtracker: make Stop safe before Start and idempotent

eth/txtracker/tracker.go

@@ -81,6 +81,7 @@ type Tracker struct {
 	sub          event.Subscription
 
 	quit     chan struct{}
+	stopOnce sync.Once
 	step     chan struct{} // test sync: sent after each event is processed
 	wg       sync.WaitGroup
 }
@@ -118,8 +119,12 @@ func (t *Tracker) NotifyPeerDrop(peer string) {
 
 // Stop shuts down the tracker.
 func (t *Tracker) Stop() {
+	t.stopOnce.Do(func() {
+		if t.sub != nil {
 			t.sub.Unsubscribe()
+		}
 		close(t.quit)
+	})
 	t.wg.Wait()
 }