'if exists' takes a parameter which can be directory, directory/,
directory/. directory\ or directory\. for example, and should equate
to true if the directory exists. The syntax directory\ is explicitly
rejected by FindFirstFile and hence was not working - look for this
specific case, and if found append a '.'.

Follow-up commit of bc9d68bc

Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=55130#add_comment