upstream: invalidate dh->priv_key after freeing it in error path;

avoids unlikely double-free later. Reported by Viktor Dukhovni via
openssh#96 feedback jsing@ tb@

OpenBSD-Commit-ID: e317eb17c3e05500ae851f279ef6486f0457c805

dh.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: dh.c,v 1.65 2018/06/26 11:23:59 millert Exp $ */
+/* $OpenBSD: dh.c,v 1.66 2018/08/04 00:55:06 djm Exp $ */
 /*
  * Copyright (c) 2000 Niels Provos.  All rights reserved.
  *
@@ -283,9 +283,8 @@ dh_gen_key(DH *dh, int need)
 	if (DH_generate_key(dh) == 0)
 		return SSH_ERR_LIBCRYPTO_ERROR;
 	DH_get0_key(dh, &pub_key, NULL);
-	if (!dh_pub_is_valid(dh, pub_key)) {
+	if (!dh_pub_is_valid(dh, pub_key))
 		return SSH_ERR_LIBCRYPTO_ERROR;
-	}
 	return 0;
 }