prevent empty locations returned

fix https://github.com/chrislusf/seaweedfs/issues/1313

weed/server/master_server_handlers.go

@@ -72,9 +72,6 @@ func (ms *MasterServer) findVolumeLocation(collection, vid string) operation.Loo
 			for _, loc := range machines {
 				locations = append(locations, operation.Location{Url: loc.Url(), PublicUrl: loc.PublicUrl})
 			}
-			if locations == nil {
-				err = fmt.Errorf("volume id %s not found", vid)
-			}
 		}
 	} else {
 		machines, getVidLocationsErr := ms.MasterClient.GetVidLocations(vid)
@@ -83,6 +80,9 @@ func (ms *MasterServer) findVolumeLocation(collection, vid string) operation.Loo
 		}
 		err = getVidLocationsErr
 	}
+	if len(locations) == 0 && err == nil {
+		err = fmt.Errorf("volume id %s not found", vid)
+	}
 	ret := operation.LookupResult{
 		VolumeId:  vid,
 		Locations: locations,