std: fix Windows junction_point reparse buffer and rollback

[SebTardif]

Summary

Write the mount-point substitute-name terminating NUL (fix uninit IOCTL input) and roll back the created directory if reparse setup fails.

Problem

1. PrintNameOffset reserved a NUL slot after the substitute name but that u16 was never written (MaybeUninit path buffer, Windows junction_point() passes uninitialized memory as null terminators #263).
2. Failure after create_new left an empty non-junction directory.

Change

library/std/src/sys/fs/windows.rs junction_point: write 0u16 after substitute name; adjust ReparseDataLength; on error drop handle and best-effort rmdir.

Origin

Lines	Commit	Date	Author
1636-1712	d6945f6d8c1	2025-07-17	ltdk

Closes #263
Closes #548