CDRIVER-745 crash in _mongoc_cluster_disconnect_node

In _mongoc_cluster_node_destroy, don't NULL the node->tags bson.
Reinitialize it instead, so it can be safely destroyed again if the
node is reused and disconnected.

Author: ajdavis

src/mongoc/mongoc-cluster.c

@@ -329,7 +329,7 @@ _mongoc_cluster_node_destroy (mongoc_cluster_node_t *node)
 
    if (node->tags.len) {
       bson_destroy (&node->tags);
-      memset (&node->tags, 0, sizeof node->tags);
+      bson_init (&node->tags);
    }
 
    bson_free (node->replSet);