Use { 0, } for structure initialization rather than memset()

It's cleaner, safer, and I had a totally wrong idea stuck in my head
about why memset() should be used.

https://bugzilla.gnome.org/show_bug.cgi?id=705968

src/libostree/ostree-repo-prune.c

@@ -103,11 +103,9 @@ ostree_repo_prune (OstreeRepo        *repo,
   gs_unref_hashtable GHashTable *objects = NULL;
   gs_unref_hashtable GHashTable *all_refs = NULL;
   gs_free char *formatted_freed_size = NULL;
-  OtPruneData data;
+  OtPruneData data = { 0, };
   gboolean refs_only = flags & OSTREE_REPO_PRUNE_FLAGS_REFS_ONLY;
 
-  memset (&data, 0, sizeof (data));
-
   data.repo = repo;
   data.reachable = ostree_repo_traverse_new_reachable ();