gkeyfilesettingsbackend: Fix basename handling when group is unset

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