[mucurses] Ensure SLK labels are always terminated

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/hci/mucurses/slk.c

 		return ERR;
 
 	strncpy(slks->fkeys[labnum].label, label,
-		sizeof(slks->fkeys[labnum].label));
+		(sizeof(slks->fkeys[labnum].label) - 1));
 	slks->fkeys[labnum].fmt = fmt;
 
 	return OK;