crypt32: Don't expect ImageGetCertificateData to succeed when Certificate is NULL

[Juan Lang]

This mirrors a fix I made to wintrust in
http://www.winehq.org/pipermail/wine-cvs/2008-January/039618.html ,
and fixes a regression introduced by
http://www.winehq.org/pipermail/wine-cvs/2007-December/038750.html
Thanks to AF for the report.
--Juan
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