Fix leak in ostree_repo_list_collection_refs
Fix leak in ostree_repo_list_collection_refs

We need to have the g_auto(GLnxDirFdIterator) inside the loop, or
we don't correctly clean up when iterating several times.

Closes: #1700
Approved by: cgwalters

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree