adjust to vmod_priv methods

Ref:
https://github.com/varnishcache/varnish-cache/commit/681c11998ec56dc4460a30585210f95a7850ea3b

src/vmod_blobdigest.c

@@ -370,7 +370,7 @@ get_scope(const struct vrt_ctx * const restrict ctx,
 		task->result = NULL;
 		priv->priv = task;
 		priv->len = sizeof(struct digest_task);
-		priv->free = NULL;
+		AZ(priv->methods);
 	}
 	return (task);
 }