CPython: tar filter allows writing outside destination via empty-named symlink

tarfile.tar_filter could be bypassed using crafted link entries, including symlinks with empty or directory-like names, to redirect later archive members outside the intended extraction directory. This allowed a malicious tar archive to cause tarfile.extractall() with filter="tar" to write files outside the destination directory, subject to the permissions of the extracting process.

This is the tarfile.tar_filter counterpart of CVE-2026-7774, which covered tarfile.data_filter.