CVE-2026-33055: tar-rs incorrectly ignores PAX size headers if header size is nonzero

As part of CVE-2025-62518 the astral-tokio-tar project was changed to correctly honor PAX size headers in the case where it was different from the base header.

However, it was missed at the time that this project (the original Rust tar crate) had a conditional logic that skipped the PAX size header in the case that the base header size was nonzero - almost the inverse of the astral-tokio-tar issue.

The problem here is that any discrepancy in how tar parsers honor file size can be used to create archives that appear differently when unpacked by different archivers.

In this case, the tar-rs (Rust tar) crate is an outlier in checking for the header size - other tar parsers (including e.g. Go archive/tar) unconditionally use the PAX size override.