Fix parsing of '0' and non-alphanumerics in steps. Previously, an

entry having stepping value of zero can cause crontab to hang there,
and if the main crontab is being changed in this way, then cron(8)
will keep spining.

Obtained from:	OpenBSD [src/usr.sbin/cron/entry.c,v 1.17]
PR:		68683 (my own, but forgot to commit it...)
MFC After:	1 week

usr.sbin/cron/lib/entry.c

@@ -543,7 +543,7 @@ get_range(bits, low, high, names, ch, file)
 		 * sent as a 0 since there is no offset either.
 		 */
 		ch = get_number(&num3, 0, PPC_NULL, ch, file);
-		if (ch == EOF)
+		if (ch == EOF || num3 == 0)
 			return EOF;
 	} else {
 		/* no step.  default==1.
@@ -592,6 +592,8 @@ get_number(numptr, low, names, ch, file)
 		ch = get_char(file);
 	}
 	*pc = '\0';
+	if (len == 0)
+	    return (EOF);
 
 	/* try to find the name in the name list
 	 */