BoundOS.abs recurses forever when baseDir is empty

[andrew]

With an empty base dir and the default deduplicatePath, abs calls itself with the same input:

osfs.New("", osfs.WithBoundOS()).Stat("/tmp")
// fatal error: stack overflow

In os_bound.go abs:

dup := filepath.Join(fs.baseDir, fs.baseDir[len(vol):])           // "" when baseDir == ""
if strings.HasPrefix(path, dup+string(filepath.Separator)) {      // HasPrefix(path, "/") -> true
    return fs.abs(path[len(dup):])                                // fs.abs(path) -> same input
}

Reproduced on v5.8.0 and v5.9.0, darwin/arm64.

Happy to send a PR. Guarding the dedup block with fs.baseDir != "" (or dup != "") fixes it.

[andrew]

The repro from this issue (osfs.New("", osfs.WithBoundOS()).Stat("/tmp")) returns cleanly at 13ded9e — #211 removed the deduplication path entirely so the recursion no longer exists. Closing as resolved by #211.