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.
void credssp_encode_ts_credentials(rdpCredssp* credssp)
{
	wStream* s;
	int length;

	length = ber_sizeof_sequence(credssp_sizeof_ts_credentials(credssp));
	sspi_SecBufferAlloc(&credssp->ts_credentials, length);

	s = Stream_New(credssp->ts_credentials.pvBuffer, length);
	credssp_write_ts_credentials(credssp, s);

	Stream_Free(s, FALSE);
}

void credssp_encode_ts_credentials(rdpCredssp* credssp)
{
	wStream* s;
	int length;

	length = ber_sizeof_sequence(credssp_sizeof_ts_credentials(credssp));
	sspi_SecBufferAlloc(&credssp->ts_credentials, length);

	s = Stream_New(credssp->ts_credentials.pvBuffer, length);
	credssp_write_ts_credentials(credssp, s);

	Stream_Free(s, FALSE);
}
