cryptnet: Sign-compare warning fix (try 2)

[Juan Lang]

Hi Andy,

> An error code like CRYPT_E_NOT_FOUND is, in essence, a LONG (signed).
> Whereas GetLastError() returns a DWORD (unsigned, which does seem
> inappropriate to me).

Yes, I know what the value of CRYPT_E_NOT_FOUND is, and what the type
of GetLastError is.  My point is, Microsoft confused signed and
unsigned types for their last error values.  We have to live with it.

> Regarding whether there is a point in removing such errors: I suppose, if we
> want to apply -Wsign-compare for the whole codebase and have zero warnings
> during a build, then I don't think we have any choice.

I don't think the cost of adding silly casts or temporaries to avoid
giving a warning on a common code idiom is worth it.
--Juan