[Bug 38796] dwrite analyzer test crashes on my system

https://bugs.winehq.org/show_bug.cgi?id=38796

--- Comment #4 from Bernhard Übelacker  ---
Even while sent patch fixes the Wine implementation, the tests do not succeed
on Windows. So Windows does not to support RegEnumValue being called with
value or val_count as NULL.

Currently RegEnumValue is also used that way in Wine itself, so fixing it right
now would lead to more problems.

Therefore trying to get something in to have these RegEnumValue covered by
tests.
Then changing these RegEnumValue uses and as last change RegEnumValue itself.