msvc: avoid calling `access("NUL", flags)`

msvc: avoid calling `access("NUL", flags)`

Apparently this is not supported with Microsoft's Universal C Runtime.
So let's not actually do that.

Instead, just return success because we _know_ that we expect the `NUL`
device to be present.

Side note: it is possible to turn off the "Null device driver" and
thereby disable `NUL`. Too many things are broken if this driver is
disabled, therefore it is not worth bothering to try to detect its
presence when `access()` is called.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>