list-deltas: Don't break on non-subdir entries

list-deltas: Don't break on non-subdir entries

ostree_repo_list_static_delta_names() tried to validate that
any second-level directory element was a directory, but there was
a cut-and-paste issue, and it used `dent->d_type` instead
of `sub_dent->d_type`.

This fixes the code, but all old ostree versions will break if
there are non-directories in a subdirectory of the deltas directory
in the repo, so be wary.