xz: Fix --files/--files0 usage via XZ_OPT and XZ_DEFAULTS env vars

xz: Fix --files/--files0 usage via XZ_OPT and XZ_DEFAULTS env vars

If --files=FILELIST or --files0=FILELIST was specified via an enrivonment
variable, it worked if there were no errors when reading from the file
FILELIST. However, if an error occurred when reading from FILELIST,
there was a use-after-free bug when printing the error message because
the memory containing the string "FILELIST" had been freed.

    printf foo.xz > filelist.txt
    XZ_OPT=--files=filelist.txt xz -l

Because filelist.txt doesn't end in a newline, the file list is seen as
invalid/truncated, resulting in an error message like this:

    xz: ????????.txt: Unexpected end of input when reading filenames

The question marks are because the garbage string is masked using
tuklib_mask_nonprint(). After this commit, it works:

    xz: filelist.txt: Unexpected end of input when reading filenames

Co-authored-by: Lasse Collin <lasse.collin@tukaani.org>
Fixes: https://github.com/tukaani-project/xz/pull/223