Safely close database connections and delete backends

Also fix NewClosure leak caused by disconnected object signals

src/subsonic/subsonicbaserequest.cpp

@@ -50,7 +50,7 @@ SubsonicBaseRequest::~SubsonicBaseRequest() {
 
   while (!replies_.isEmpty()) {
     QNetworkReply *reply = replies_.takeFirst();
-    disconnect(reply, 0, nullptr, 0);
+    disconnect(reply, 0, this, 0);
     if (reply->isRunning()) reply->abort();
     reply->deleteLater();
   }