pull: Fix a used before initialization warning

src/libostree/ostree-repo-pull.c:1676:22: warning: 'flags' may be used uninitialized in this function [-Wmaybe-uninitialized]

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
diff: Use gs_unref_object not gs_free on a GFile

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>