SubMirror now works again against old PuSH

There was a problem with (specifically at least) PuSHpress for
Wordpress. A previous attempt to perform a DB transaction backfired
because the remote side could connect to the callback before our
commit had gone through.

I take full responsibility for introducing the bug in the first place :)

plugins/OStatus/actions/pushcallback.php

@@ -52,7 +52,7 @@ class PushCallbackAction extends Action
         }
 
         $feedsub = FeedSub::getKV('id', $feedid);
-        if (!$feedsub) {
+        if (!$feedsub instanceof FeedSub) {
             // TRANS: Server exception. %s is a feed ID.
             throw new ServerException(sprintf(_m('Unknown PuSH feed id %s'),$feedid), 400);
         }