strptime: disallow zero hour for %I (defined by POSIX as [01,12]) and %l

(extension, defined in strftime(3) as 1-12).

Approved by:	re (gjb), kib (mentor)
Differential Revision:	https://reviews.freebsd.org/D17543

lib/libc/stdtime/strptime.c

@@ -291,7 +291,7 @@ label:
 			if (c == 'H' || c == 'k') {
 				if (i > 23)
 					return (NULL);
-			} else if (i > 12)
+			} else if (i == 0 || i > 12)
 				return (NULL);
 
 			tm->tm_hour = i;