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.
SECURITY_STATUS SEC_ENTRY ImpersonateSecurityContext(PCtxtHandle phContext)
{
	return SEC_E_OK;
}

SECURITY_STATUS SEC_ENTRY ImpersonateSecurityContext(PCtxtHandle phContext)
{
	return SEC_E_OK;
}
