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/ostree/ot-builtin-checksum.c

@@ -61,9 +61,7 @@ ostree_builtin_checksum (int argc, char **argv, GFile *repo_path_path, GCancella
   GOptionContext *context;
   gboolean ret = FALSE;
   gs_unref_object GFile *f = NULL;
-  AsyncChecksumData data;
-
-  memset (&data, 0, sizeof (data));
+  AsyncChecksumData data = { 0, };
 
   context = g_option_context_new ("FILENAME - Checksum a file or directory");
   g_option_context_add_main_entries (context, options, NULL);