set sshpam_ctxt to NULL after free

Avoids use-after-free in monitor when privsep child is compromised.
Reported by Moritz Jodeit; ok dtucker@
monitor_child_postauth(struct monitor *pmonitor)
{
	close(pmonitor->m_recvfd);
	pmonitor->m_recvfd = -1;

	monitor_set_child_handler(pmonitor->m_pid);
	signal(SIGHUP, &monitor_child_handler);
	signal(SIGTERM, &monitor_child_handler);
	signal(SIGINT, &monitor_child_handler);
#ifdef SIGXFSZ
	signal(SIGXFSZ, SIG_IGN);
#endif

	if (compat20) {
		mon_dispatch = mon_dispatch_postauth20;

		/* Permit requests for moduli and signatures */
		monitor_permit(mon_dispatch, MONITOR_REQ_MODULI, 1);
		monitor_permit(mon_dispatch, MONITOR_REQ_SIGN, 1);
		monitor_permit(mon_dispatch, MONITOR_REQ_TERM, 1);
	} else {
		mon_dispatch = mon_dispatch_postauth15;
		monitor_permit(mon_dispatch, MONITOR_REQ_TERM, 1);
	}
	if (!no_pty_flag) {
		monitor_permit(mon_dispatch, MONITOR_REQ_PTY, 1);
		monitor_permit(mon_dispatch, MONITOR_REQ_PTYCLEANUP, 1);
	}

	for (;;)
		monitor_read(pmonitor, mon_dispatch, NULL);
}

monitor_child_postauth(struct monitor *pmonitor)
{
	close(pmonitor->m_recvfd);
	pmonitor->m_recvfd = -1;

	monitor_set_child_handler(pmonitor->m_pid);
	signal(SIGHUP, &monitor_child_handler);
	signal(SIGTERM, &monitor_child_handler);
	signal(SIGINT, &monitor_child_handler);
#ifdef SIGXFSZ
	signal(SIGXFSZ, SIG_IGN);
#endif

	if (compat20) {
		mon_dispatch = mon_dispatch_postauth20;

		/* Permit requests for moduli and signatures */
		monitor_permit(mon_dispatch, MONITOR_REQ_MODULI, 1);
		monitor_permit(mon_dispatch, MONITOR_REQ_SIGN, 1);
		monitor_permit(mon_dispatch, MONITOR_REQ_TERM, 1);
	} else {
		mon_dispatch = mon_dispatch_postauth15;
		monitor_permit(mon_dispatch, MONITOR_REQ_TERM, 1);
	}
	if (!no_pty_flag) {
		monitor_permit(mon_dispatch, MONITOR_REQ_PTY, 1);
		monitor_permit(mon_dispatch, MONITOR_REQ_PTYCLEANUP, 1);
	}

	for (;;)
		monitor_read(pmonitor, mon_dispatch, NULL);
}
