Fix an effective regression in commit
7781a9cb, which happens when
`convert_path()` is called with a `key` which contains no slashes. In
that case, the `key` is entirely the `basename`.

Prior to commit 7781a9cb, the code worked through a fluke of `i == -1`
cancelling out with the various additions in the `g_memdup()` call, and
effectively resulting in `g_strdup (key)`.

Spotted by Guido Berhoerster.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>