shlwapi: Don't bother trying to duplicate nil handle

[Bruno Jesus]

On Wed, Nov 25, 2015 at 3:17 PM, Olivier F. R. Dierick
<o.dierick at piezo-forte.be> wrote:
> Fix one of the error leak source that prevents the installer to succeed
> (bug 36838).
>
> The original code did try to duplicate the handle even though it is
> null, and that causes last error to be set to ERROR_INVALID_HANDLE. The
> game installer chokes at some point when the last error is anything but
> zero and fails to complete the installation. Returning a null handle
> value is sufficient to indicate that error condition. The SHMapHandle is
> not supposed to set last error on failure.

A test wouldn't hurt, like the attached patch. Maybe to make this more
convincing the test should be sent separate to prove current wine
behavior is wrong.
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