shlwapi: fix UrlUnEscape inplace behavior (2/2) (try 2)

[Dan Kegel]

Fix UrlUnEscape to expand URLs inplace even if the output buffer is NULL.
This is an important part of fixing Lively's installer,
http://bugs.winehq.org/show_bug.cgi?id=14381

Changes since Louis originally posted as
http://www.winehq.org/pipermail/wine-patches/2008-August/059445.html
1) removes todo_wine around tests it fixes
2) Louis requested that somebody else take over these patches
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