Describe the bug
--output-dir-mirror fails to compress files with names containing ".."

This also applies to directories containing ".." in their names.

To Reproduce

touch a..b
zstd a..b --output-dir-mirror output

Produces the error zstd: --output-dir-mirror cannot compress 'a..b' into 'output'

Expected behavior
"a..b" is compressed as "output/a..b.zst"

Desktop (please complete the following information):

• OS: Fedora Linux
• Version: 1.4.7