bugfix(file): Keep the resolved prefix when a write path has a missing directory

[bobtista]

• Closes Writing through a directory that does not exist yet creates the file in the working directory on non-Windows builds #3099

On an unmatched component during a write the fixup does pathFixed = p, replacing everything matched so far, and the following pathFixed /= pathFixedPart appends nothing because pathFixedPart is empty:

requested:  /home/user/GeneralsData/Maps/MyMap/MyMap.tga
before:     MyMap.tga
after:      /home/user/GeneralsData/Maps/MyMap/MyMap.tga

Now the component is appended to the resolved prefix instead of replacing it. The comment above the line already said to use the last known good path.

Todo:

• A file written through a directory that does not exist yet is created at the requested path
• Replicate to Generals — N/A, it's in Core