xz: Fix suffix check.

xz: Fix suffix check.

The suffix refactor done in 99575947a58a60416c570eb78038d18a1ea4cef4
had a small regression where raw format compression to standard out
failed if a suffix was not set. In this case, setting the suffix did
not make sense since a file is not created.

Now, xz should only fail when a suffix is not provided when it is
actually needed.

For instance:

    echo "foo" | xz --format=raw --lzma2 | wc -c

does not need a suffix check since it creates no files. But:

    xz --format=raw --lzma2 --suffix=.bar foo

Needs the suffix to be set since it must create foo.bar.