Fix Flexihash constructor

lib/blogs.php

@@ -32,7 +32,7 @@ function noblogs_get_backend_for_blog($blog_id) {
   // Create a temporary hash object just to load the backends
   // (we cannot use the map in $wpdb because it doesn't have the
   // backend array anymore).
-  $hashmap = new Flexihash();
+  $hashmap = new Flexihash(null, R2DB_FLEXIHASH_REPLICAS);
   $reversemap = noblogs_load_backends(NOBLOGS_BACKEND_CONFIG, $hashmap);
 
   // Lookup the blog ID using the $wpdb hash just to be safe (though