TJClientFile::getDefaultPath fix against "invalid next size"

Adrian Sevcencorequested to merge
asevcenc/jalien-root:fix_bug into master

For a number of my jobs on the grid i got free(): invalid next size (fast). See also https://root-forum.cern.ch/t/root-exe-free-invalid-next-size-fast/47399 This patch rewrites the getDefaultPath in a simple and without conversions way

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