lib/repo-pull: Add a missing precondition

This catches a few failure modes in the pull code a little earlier,
before the incorrectly-NULL repo makes its way into a closure and a
worker thread, where the cause of the problem is harder to track down.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #1058
Approved by: cgwalters

src/libostree/ostree-repo-pull.c

@@ -3270,6 +3270,7 @@ ostree_repo_pull_with_options (OstreeRepo             *self,
       (void) g_variant_lookup (options, "localcache-repos", "^a&s", &opt_localcache_repos);
     }
 
+  g_return_val_if_fail (OSTREE_IS_REPO (self), FALSE);
   g_return_val_if_fail (pull_data->maxdepth >= -1, FALSE);
   g_return_val_if_fail (!opt_collection_refs_set ||
                         (refs_to_fetch == NULL && override_commit_ids == NULL), FALSE);