extractor: Fix extraction in symlinked locations

The "The file is not a directory" error is returned in case the archive has
common prefix, which have not been changed over the `decide-destination`
signal and the destination is a symlink. This is regression caused by the
commit b9590ab7. It causes that the security checks, which was added by the
commit 8109c368, verifies also the destination directory by mistake. Let's
partially rework the problematic commit to avoid this regression.

Fixes: https://gitlab.gnome.org/GNOME/gnome-autoar/-/issues/36