fix make vet failures, and enable make vet on CI

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

cmd/ctr/events.go

@@ -42,6 +42,5 @@ var eventsCommand = cli.Command{
 				return err
 			}
 		}
-		return nil
 	},
 }