Cache bug in _sitemenu_overview()

When reading from the cache, _sitemenu_overview() tries to do this using

  $tree_nodes = unserialize(cache_get("sitemenu:tree_nodes_$type". $vocabulary->vid . $mode));

but this won't work since it is the ->data property of cache_get() that must be unserialized, not the array returned by cache_get() itself. (Generates a notice when E_ALL is on.) For example, menu.inc does this:

    if ($cached = cache_get($cid)) {
      $_menu = unserialize($cached->data);
    }

The attached patch attempts to fix sitemenu.module in the same way.

NB: I have tested the fix on my local 4.7.3 installation, but the patch is generated from CVS version and has not been tested. Sorry about that. Mark.