core/image: fix usage of "unknown" platform

"unknown" should not be returned as a valid platform
supported by the image

Signed-off-by: Akhil Mohan <akhilerm@gmail.com>

images/image.go

@@ -268,6 +268,9 @@ func Platforms(ctx context.Context, provider content.Provider, image ocispec.Des
 	var platformSpecs []ocispec.Platform
 	return platformSpecs, Walk(ctx, Handlers(HandlerFunc(func(ctx context.Context, desc ocispec.Descriptor) ([]ocispec.Descriptor, error) {
 		if desc.Platform != nil {
+			if desc.Platform.OS == "unknown" || desc.Platform.Architecture == "unknown" {
+				return nil, ErrSkipDesc
+			}
 			platformSpecs = append(platformSpecs, *desc.Platform)
 			return nil, ErrSkipDesc
 		}