txn nil guard when rolling back LatestEventsUpdater (#1524)

* txn nil guard when rolling back LatestEventsUpdater

* Spell lint correctly

roomserver/storage/shared/latest_events_updater.go

@@ -18,23 +18,30 @@ type LatestEventsUpdater struct {
 	currentStateSnapshotNID types.StateSnapshotNID
 }
 
+func rollback(txn *sql.Tx) {
+	if txn == nil {
+		return
+	}
+	txn.Rollback() // nolint: errcheck
+}
+
 func NewLatestEventsUpdater(ctx context.Context, d *Database, txn *sql.Tx, roomInfo types.RoomInfo) (*LatestEventsUpdater, error) {
 	eventNIDs, lastEventNIDSent, currentStateSnapshotNID, err :=
 		d.RoomsTable.SelectLatestEventsNIDsForUpdate(ctx, txn, roomInfo.RoomNID)
 	if err != nil {
-		txn.Rollback() // nolint: errcheck
+		rollback(txn)
 		return nil, err
 	}
 	stateAndRefs, err := d.EventsTable.BulkSelectStateAtEventAndReference(ctx, txn, eventNIDs)
 	if err != nil {
-		txn.Rollback() // nolint: errcheck
+		rollback(txn)
 		return nil, err
 	}
 	var lastEventIDSent string
 	if lastEventNIDSent != 0 {
 		lastEventIDSent, err = d.EventsTable.SelectEventID(ctx, txn, lastEventNIDSent)
 		if err != nil {
-			txn.Rollback() // nolint: errcheck
+			rollback(txn)
 			return nil, err
 		}
 	}