[BT] Fix TFMB::FindIndex() for empty paths.

Just immediately return an invalid index, as empty parent path is root path.

Somewhat for #1736.
[BT] Slightly more idiot-proof^WW usable.

If original torrent file path contains '/', and the new one doesn't, just
change the file name to the new one (that is, after renaming `foo/bar/baz`
to `qux` the actual full path will be `foo/bar/quz`).

Finally fixes #1736.