RemoteComponent: add apparently missing locking

It accesses m_subscriber, so it definitely should lock m_subscriberLock.

handleSubscription() also acquires the lock (and it is not recursive),
so there is a logical race condition left. However, this shouldn't cause
any real problems, because the code below can deal with a subscriber
being removed during the race condition window.

src/remote/RemoteComponent.cpp

@@ -219,8 +219,13 @@ void RemoteComponent::handleCommand(QHttpRequest* request, QHttpResponse* respon
   }
   else if ((request->url().path() == "/player/timeline/poll"))
   {
+    QMutexLocker lk(&m_subscriberLock);
     if (!m_subscriberMap.contains(identifier))
+    {
+      lk.unlock();
       handleSubscription(request, response, true);
+      lk.relock();
+    }
 
     RemotePollSubscriber *subscriber = (RemotePollSubscriber *)m_subscriberMap[identifier];
     if (subscriber)