services: fix defer function in retry of NeoFSBlockFetcher

Drain the channel only if it has pending events. This avoids trying to
read from an already-empty channel.

Close #3734

Signed-off-by: Ekaterina Pavlova <ekt@morphbits.io>

pkg/services/blockfetcher/blockfetcher.go

@@ -518,7 +518,10 @@ func (bfs *Service) retry(action func() error) error {
 	)
 	defer func() {
 		if !timer.Stop() {
-			<-timer.C
+			select {
+			case <-timer.C:
+			default:
+			}
 		}
 	}()