cli: avoid nil dereference in silence update (#2427)

Signed-off-by: Koki Kato <koki.kato1994@gmail.com>

cli/silence_update.go

@@ -64,10 +64,10 @@ func (c *silenceUpdateCmd) update(ctx context.Context, _ *kingpin.ParseContext)
 		params := silence.NewGetSilenceParams()
 		params.SilenceID = strfmt.UUID(silenceID)
 		response, err := amclient.Silence.GetSilence(params)
-		sil := response.Payload
 		if err != nil {
 			return err
 		}
+		sil := response.Payload
 		if c.start != "" {
 			startsAtTime, err := time.Parse(time.RFC3339, c.start)
 			if err != nil {