Fix possible infinite loop in qBtSession constructor (when Web UI is enabled)

src/mainwindow.cpp

@@ -331,6 +331,7 @@ MainWindow::~MainWindow() {
   // Delete BTSession objects
   qDebug("Deleting BTSession");
   QBtSession::drop();
+  BTSession = 0;
   // May freeze for a few seconds after the next line
   // because the Bittorrent session proxy will
   // actually be deleted now and destruction