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 sspi_SecBufferFree(PSecBuffer SecBuffer)
{
	free(SecBuffer->pvBuffer);
	SecBuffer->pvBuffer = NULL;
	SecBuffer->cbBuffer = 0;
}

void sspi_SecBufferFree(PSecBuffer SecBuffer)
{
	free(SecBuffer->pvBuffer);
	SecBuffer->pvBuffer = NULL;
	SecBuffer->cbBuffer = 0;
}
