crypt32: Fix length-handling of 'decodeBase64Block'

[Marvin]

Hi,

While running your changed tests on Windows, 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
http://testbot.winehq.org/JobDetails.pl?Key=10497

Your paranoid android.


=== WINEBUILD (build) ===
Patch failed to apply