Don't fail when apr and apr-util only have Release mode built.

* build/generator/gen_win_dependencies.py
  (GenDependenciesBase._find_apr,
   GenDependenciesBase._find_apr_util_and_expat): Check that the directory
    exists before trying to get the files in the directory.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1560558 13f79535-47bb-0310-9956-ffa450edef68