oauthutil: fix nil pointer crash when started with expired token

lib/oauthutil/renew.go

@@ -88,7 +88,9 @@ func (r *Renew) Shutdown() {
 	}
 	// closing a channel can only be done once
 	r.shutdown.Do(func() {
+		if r.ts != nil {
 			r.ts.expiryTimer.Stop()
+		}
 		close(r.done)
 	})
 }