upstream: fix bogus warning when signing cert keys using agent;

from djm; ok deraadt dtucker

OpenBSD-Commit-ID: 12e50836ba2040042383a8b71e12d7ea06e9633d
diff --git a/sshconnect2.c b/sshconnect2.c
index 2f0a57f..1f4a74c 100644
--- a/sshconnect2.c
+++ b/sshconnect2.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: sshconnect2.c,v 1.269 2018/03/03 03:01:50 djm Exp $ */
+/* $OpenBSD: sshconnect2.c,v 1.270 2018/03/24 19:28:43 markus Exp $ */
 /*
  * Copyright (c) 2000 Markus Friedl.  All rights reserved.
  * Copyright (c) 2008 Damien Miller.  All rights reserved.
@@ -1014,6 +1014,8 @@
 	char *sigtype = NULL;
 	const char *alg = key_sign_encode(key);
 
+	if (sshkey_is_cert(key))
+		return 0;
 	if ((r = sshkey_sigtype(sig, len, &sigtype)) != 0)
 		return r;
 	if (strcmp(sigtype, alg) != 0) {