plug a memory leak if .final is used in init/fini

src/vmod_blobdigest.c

@@ -369,7 +369,7 @@ vmod_digest_final(VRT_CTX, struct vmod_blobdigest_digest *h)
 		b->priv = malloc(digestsz);
 		AN(b->priv);
 		b->len = digestsz;
-		b->free = NULL;
+		b->free = free;
 		final(hash, &h->ctx, b->priv);
 		h->result = b;
 		return b;