schema1: Validate descriptors in AppendReference

If the digest is malformed, it will cause a panic when building the
manifest.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

manifest/schema1/config_builder.go

@@ -240,8 +240,13 @@ func (mb *configManifestBuilder) emptyTar(ctx context.Context) (digest.Digest, e
 
 // AppendReference adds a reference to the current ManifestBuilder
 func (mb *configManifestBuilder) AppendReference(d distribution.Describable) error {
-	// todo: verification here?
-	mb.descriptors = append(mb.descriptors, d.Descriptor())
+	descriptor := d.Descriptor()
+
+	if err := descriptor.Digest.Validate(); err != nil {
+		return err
+	}
+
+	mb.descriptors = append(mb.descriptors, descriptor)
 	return nil
 }