Freak out in a controlled way if this happens.

git-svn-id: svn+ssh://src.earth.threerings.net/vilya/trunk@482 c613c5cb-e716-0410-b11b-feb51c14d237

src/as/com/threerings/whirled/client/SceneDirector.as

@@ -215,6 +215,11 @@ public class SceneDirector extends BasicDirector
         clearScene();
 
         // make the pending scene the active scene
+        if (_pendingData == null) {
+            log.warning("Aiya! Scene move succeeded but we have no pending data! " +
+                        "[prevId=" + _previousSceneId + "].");
+            return;
+        }
         _sceneId = _pendingData.sceneId;
         _pendingData = null;