server: abort download on empty digest

Signed-off-by: Kyle Steere <kyle.steere@chainguard.dev>

server/download.go

@@ -464,6 +464,10 @@ type downloadOpts struct {
 
 // downloadBlob downloads a blob from the registry and stores it in the blobs directory
 func downloadBlob(ctx context.Context, opts downloadOpts) (cacheHit bool, _ error) {
+	if opts.digest == "" {
+		return false, fmt.Errorf(("%s: %s"), opts.mp.GetNamespaceRepository(), "digest is is empty")
+	}
+
 	fp, err := GetBlobsPath(opts.digest)
 	if err != nil {
 		return false, err