msi: make the WriteEnvironmentStrings handling of [~] a bit better

[Mikolaj Zalewski]

EA Durbin wrote:
> The compiler complains at this line about lstrlenW being passed an incompatible pointer type
>
> +    else if (lstrlenW(value) >= prefix_len)
>
>
>
> should it be lstrlenW(*value) ?
>   

Thanks for noticing it.

Mikołaj Zalewski