msi: Uninitialized variable fix (Coverity)

[Juan Lang]

Hi Andy,

> Sorry, I don't understand what I have done wrong. RegCloseKey() will return ERROR_INVALID_HANDLE if called with hkey==NULL.

To expand on James's brief response,

> -    HKEY userdata;
> +    HKEY userdata = NULL;

This change looks correct, but the other isn't necessary.
--Juan