SecureJoin: on windows, paths outside of the rootfs could be inadvertently produced

### Impact For Windows users of `github.com/cyphar/filepath-securejoin`, until v0.2.4 it was possible for certain rootfs and path combinations (in particular, where a malicious Unix-style `/`-separated unsafe path was used with a Windows-style rootfs path) to result in generated paths that were outside of the provided rootfs.

It is unclear to what extent this has a practical impact on real users, but given the possible severity of the issue we have released an emergency patch release that resolves this issue.

Thanks to @pjbgf for discovering, debugging, and fixing this issue (as well as writing some tests for it).

### Patches c121231e1276e11049547bee5ce68d5a2cfe2d9b is the patch fixing this issue. v0.2.4 contains the fix.

### Workarounds Users could use `filepath.FromSlash()` on all unsafe paths before passing them to `filepath-securejoin`.

### References See #9.