advapi32: Fix RegEnumValueW when enumerating long values.

[Bernhard Übelacker]

Hello,
please ignore this patch for now.

It seems that systemfontfileenumerator_GetCurrentFontFile uses 
RegEnumValueW in a way not supported on Windows - with giving NULL 
parameters for value and/or val_count [1].

When RegEnumValueW is called with proper value parameter the problem 
with not returning correct count would not happen already.

So, as far as I see, first all uses of RegEnumValue with NULL parameters 
for value or val_count need a fix.

Kind regards,
Bernhard

[1] https://testbot.winehq.org/JobDetails.pl?Key=14533