[PATCH] ntdll: Fix a MSVC amd64 C4334 warning

[Marvin]

Hi,

While running your changed tests, I think I found new failures.
Being a bot and all I'm not very good at pattern recognition, so I might be
wrong, but could you please double-check?

Full results can be found at:
https://testbot.winehq.org/JobDetails.pl?Key=88926

Your paranoid android.


=== debiant2 (32 bit report) ===

ntdll:
threadpool.c:1960: Test failed: WaitForSingleObject returned 258