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.
BOOL nla_verify_header(wStream* s)
{
	if ((s->pointer[0] == 0x30) && (s->pointer[1] & 0x80))
		return TRUE;

	return FALSE;
}

BOOL nla_verify_header(wStream* s)
{
	if ((s->pointer[0] == 0x30) && (s->pointer[1] & 0x80))
		return TRUE;

	return FALSE;
}
