Move get_tmp_dir to win32-util.c and error out on failure

Move get_tmp_dir to win32-util.c and error out on failure

Currently we only warn in get_tmp_dir fails and set o->tmp_dir to
a null pointer. This will not be caught by check_file_access_chroot
either since that ignores NULL pointers but other parts of OpenVPN
will assume that tmp_dir is set to a non-NULL string.

Also move get_tmp_dir to win32-util.c to use it in unit tests.

Change-Id: I525ccf7872880367b248ebebb0ddc83551498042
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20240108171349.15871-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg27964.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>