upstream: ssh: use sentinel idiom for timegm(3) and mktime(3)

There is nothing wrong with times before the epoch, even -1, so use the
idiom recently added to the CAVEATS section to figure out whether there
was an error in the timegm() or mktime() calls.

We should sweep the tree for this. If anyone is bored, feel free to beat
me to it...

ok deraadt djm

OpenBSD-Commit-ID: e2b1721966dc782e776db5d6cfb18958534f9d4b

misc.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: misc.c,v 1.213 2026/03/03 09:57:25 dtucker Exp $ */
+/* $OpenBSD: misc.c,v 1.214 2026/05/27 13:54:15 tb Exp $ */
 /*
  * Copyright (c) 2000 Markus Friedl.  All rights reserved.
  * Copyright (c) 2005-2020 Damien Miller.  All rights reserved.
@@ -2588,10 +2588,12 @@ parse_absolute_time(const char *s, uint64_t *tp)
 	if ((cp = strptime(buf, fmt, &tm)) == NULL || *cp != '\0')
 		return SSH_ERR_INVALID_FORMAT;
 	if (is_utc) {
-		if ((tt = timegm(&tm)) < 0)
+		tm.tm_wday = -1;
+		if ((tt = timegm(&tm)) == -1 && tm.tm_wday == -1)
 			return SSH_ERR_INVALID_FORMAT;
 	} else {
-		if ((tt = mktime(&tm)) < 0)
+		tm.tm_wday = -1;
+		if ((tt = mktime(&tm)) == -1 && tm.tm_wday == -1)
 			return SSH_ERR_INVALID_FORMAT;
 	}
 	/* success */