avoid uninitialised variable use found with smatch, ok tb@

sbin/iked/ca.c

···
-/*	$OpenBSD: ca.c,v 1.104 2025/09/04 10:55:19 yasuoka Exp $	*/
+/*	$OpenBSD: ca.c,v 1.105 2025/09/08 10:18:23 jsg Exp $	*/
 
 /*
  * Copyright (c) 2010-2013 Reyk Floeter <[e-mail]>
···
 	const char		*errstr = "failed";
 	X509_NAME		*subj;
 	char			*subj_name;
-	int			 depth;
+	int			 depth = -1;
 
 	if (issuerp)
 		*issuerp = NULL;