storageImageDestination: only add a digested reference to named images

When writing an image that has an instanceDigest value (meaning it's a
secondary image), don't try to generate a canonical reference to add to
the image's list of names if the reference for the primary image doesn't
contain a name.  That should only happen if we're writing using just an
image ID, which is unlikely, but we still need to handle it.

Signed-off-by: Nalin Dahyabhai <[email protected]>

storage/storage_image.go

@@ -579,7 +579,7 @@ func (s *storageImageDestination) commitDataBlobs(imageID string) error {
 func (s *storageImageDestination) commitName(imageID string, oldNames []string, instanceDigest *digest.Digest) error {
 	if name := s.imageRef.DockerReference(); len(oldNames) > 0 || name != nil {
 		var err error
-		if instanceDigest != nil {
+		if instanceDigest != nil && name != nil {
 			name, err = reference.WithDigest(reference.TrimNamed(name), *instanceDigest)
 			if err != nil {
 				return errors.Wrapf(err, "error computing name for image %q", imageID)