nla: invalidate sec handle after creation

If sec pointer isn't invalidated after creation it is not possible
to check if the upper and lower pointers are valid.

This fixes a segfault in the server part if the client disconnects before
the authentication was finished.
static int freerdp_peer_send_channel_data(freerdp_peer* client, int channelId, BYTE* data, int size)
{
	return rdp_send_channel_data(client->context->rdp, channelId, data, size);
}

static int freerdp_peer_send_channel_data(freerdp_peer* client, int channelId, BYTE* data, int size)
{
	return rdp_send_channel_data(client->context->rdp, channelId, data, size);
}
