Fix snapshotter getter in client code

Fixes #3312

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

image.go

@@ -108,7 +108,10 @@ func (i *image) Config(ctx context.Context) (ocispec.Descriptor, error) {
 }
 
 func (i *image) IsUnpacked(ctx context.Context, snapshotterName string) (bool, error) {
-	sn := i.client.SnapshotService(snapshotterName)
+	sn, err := i.client.getSnapshotter(snapshotterName)
+	if err != nil {
+		return false, err
+	}
 	cs := i.client.ContentStore()
 
 	diffs, err := i.i.RootFS(ctx, cs, i.platform)
@@ -140,13 +143,16 @@ func (i *image) Unpack(ctx context.Context, snapshotterName string) error {
 	}
 
 	var (
-		sn = i.client.SnapshotService(snapshotterName)
 		a  = i.client.DiffService()
 		cs = i.client.ContentStore()
 
 		chain    []digest.Digest
 		unpacked bool
 	)
+	sn, err := i.client.getSnapshotter(snapshotterName)
+	if err != nil {
+		return err
+	}
 	for _, layer := range layers {
 		unpacked, err = rootfs.ApplyLayer(ctx, layer, chain, sn, a)
 		if err != nil {