Security: Fix resolving of some relative paths

Security: Fix resolving of some relative paths

Trying to resolve a path with RemoveRelativePathComponents that can't
be resolved without leaving leading parent components (e.g. "../worlds/foo"
or "bar/../../worlds/foo") will fail.  To work around this, we leave
the relative components and simply remove the trailing components one
at a time, and bail out when we find a parent component.  This will
still fail for paths like "worlds/foo/noexist/../auth.txt" (the path
before the last parent component must not exist), but this is fine
since you won't be able to open a file with a path like that anyways
(the O.S. will determine that the path doesn't exist.
Try `cat /a/../etc/passwd`).