Prevent crash if likes relationship hasn't been loaded

This could theoretically happen if a post is loaded in some other context
(without its likes relationship) and overwrites the full version in the
store. While this should ideally never happen, it's better to fail
gracefully if it does.

Fixes flarum/core#1665

extensions/likes/js/src/forum/addLikeAction.js

@@ -9,7 +9,9 @@ export default function() {
 
     if (post.isHidden() || !post.canLike()) return;
 
-    let isLiked = app.session.user && post.likes().some(user => user === app.session.user);
+    const likes = post.likes();
+
+    let isLiked = app.session.user && likes && likes.some(user => user === app.session.user);
 
     items.add('like',
       Button.component({