On Windows, UTF-8 is never current locale encoding; so using 8-bit
strings will always fail for paths containing characters outside
of current codepage.
Also fix leaks caused by failing to release its result: previously
it could return either result of getenv (that shouldn't get freed),
or an allocated string, but never got freed; now the result is
always allocated and properly freed.

Change-Id: I8b255dea20040eec0572de2b34280749fe8f071c
Reviewed-on: https://gerrit.libreoffice.org/42743Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>