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_SecureHandleGetUpperPointer(SecHandle* handle)
 {
 	void* pointer;
 
	if (!handle || !SecIsValidHandle(handle))
 		return NULL;
 
 	pointer = (void*) ~((size_t) handle->dwUpper);

	return pointer;
}

void* sspi_SecureHandleGetUpperPointer(SecHandle* handle)
 {
 	void* pointer;
 
	if (!handle)
 		return NULL;
 
 	pointer = (void*) ~((size_t) handle->dwUpper);

	return pointer;
}
