FIX: Randomly failing QUnit test.

The `post` variable can be an actual post object or a `new Placeholder("post-placeholder")` which does not define the function `get`.

* QUNIT_SEED=11414431645131211212599424733847938795

app/assets/javascripts/discourse/controllers/topic.js.es6

@@ -305,7 +305,7 @@ export default Ember.Controller.extend(BufferedContent, {
       const postStream = this.get("model.postStream");
       const firstLoadedPost = postStream.get("posts.firstObject");
 
-      if (post.get("post_number") === 1) {
+      if (post.get && post.get("post_number") === 1) {
         return;
       }