fix: check snapshot labels to avoid panic

I saw a panic while using the stargz snapshotter caused
by the labels being nil.

This adds a check to prevent the panic.

Signed-off-by: Chris Goller <[email protected]>

Author: goller

cache/refs.go

@@ -1099,8 +1099,10 @@ func (sr *immutableRef) prepareRemoteSnapshotsStargzMode(ctx context.Context, s
 					if err == nil { // usable as remote snapshot without unlazying.
 						defer func() {
 							// Remove tmp labels appended in this func
-							for k := range tmpLabels {
-								info.Labels[k] = ""
+							if info.Labels != nil {
+								for k := range tmpLabels {
+									info.Labels[k] = ""
+								}
 							}
 							if _, err := r.cm.Snapshotter.Update(ctx, info, tmpFields...); err != nil {
 								bklog.G(ctx).Warn(errors.Wrapf(err,