POST_expando: gracefully fail when called on link with no child.

Previously if POST_expando was called on a link that has no link_child
(i.e. no selftext or media object to show in the expando) it would cause
an exception.

This catches the bad case early and aborts with a 404.

r2/r2/controllers/api.py

@@ -3162,7 +3162,10 @@ class ApiController(RedditController, OAuth2ResourceController):
 
         wrapped = wrap_links(link)
         wrapped = list(wrapped)[0]
-        return websafe(spaceCompress(wrapped.link_child.content()))
+        link_child = wrapped.link_child
+        if not link_child:
+            abort(404, 'not found')
+        return websafe(spaceCompress(link_child.content()))
 
     @validatedForm(VUser('password', default=''),
                    VModhash(),