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_buffer_free(rdpCredssp* credssp)
{
	sspi_SecBufferFree(&credssp->negoToken);
	sspi_SecBufferFree(&credssp->pubKeyAuth);
	sspi_SecBufferFree(&credssp->authInfo);
}

void credssp_buffer_free(rdpCredssp* credssp)
{
	sspi_SecBufferFree(&credssp->negoToken);
	sspi_SecBufferFree(&credssp->pubKeyAuth);
	sspi_SecBufferFree(&credssp->authInfo);
}
