imagetools: handle manifest with nil platform

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

util/imagetools/printers.go

@@ -59,8 +59,10 @@ func NewPrinter(ctx context.Context, opt Opt, name string, format string) (*Prin
 	switch manifest.MediaType {
 	case images.MediaTypeDockerSchema2ManifestList, ocispecs.MediaTypeImageIndex:
 		for _, m := range index.Manifests {
+			if m.Platform != nil {
 				pforms = append(pforms, *m.Platform)
 			}
+		}
 	default:
 		pforms = append(pforms, platforms.DefaultSpec())
 	}