s3: avoid nil response

fix https://github.com/chrislusf/seaweedfs/issues/2636

weed/s3api/s3api_object_multipart_handlers.go

@@ -163,13 +163,13 @@ func (s3a *S3ApiServer) ListObjectPartsHandler(w http.ResponseWriter, r *http.Re
 		UploadId:         aws.String(uploadID),
 	})
 
-	glog.V(2).Infof("ListObjectPartsHandler %s count=%d", string(s3err.EncodeXMLResponse(response)), len(response.Part))
-
 	if errCode != s3err.ErrNone {
 		s3err.WriteErrorResponse(w, r, errCode)
 		return
 	}
 
+	glog.V(2).Infof("ListObjectPartsHandler %s count=%d", string(s3err.EncodeXMLResponse(response)), len(response.Part))
+
 	writeSuccessResponseXML(w, r, response)
 
 }