[Windows] cpathCanonicalise: doesn't set length correctly

[BraedonWooding]

https://github.com/BraedonWooding/cpath/blob/ff6bfae65b4ae05dc3be838df9abe6b078ef15ee/cpath.h#L1064

Sets the input path.

Also I'm not entirely sure why we are handling the case of non-restrict, I doubt that'll apply to both windows & linux so we can likely skip a copy on one of those cases.

Likely other places too.