[staticanalysis]: Fix two uses of uninitialized variables

https://bugzilla.gnome.org/show_bug.cgi?id=732020

src/libostree/ostree-repo-traverse.c

@@ -244,7 +244,7 @@ ostree_repo_traverse_commit (OstreeRepo      *repo,
                              GCancellable    *cancellable,
                              GError         **error)
 {
-  gboolean ret;
+  gboolean ret = FALSE;
   gs_unref_hashtable GHashTable *ret_reachable =
     ostree_repo_traverse_new_reachable ();