Don't touch qbtsession here.

src/qtlibtorrent/qbtsession.cpp

@@ -402,7 +402,8 @@ void QBtSession::configureSession() {
   sessionSettings.announce_to_all_trackers = announce_to_all;
   sessionSettings.announce_to_all_tiers = announce_to_all;
   sessionSettings.auto_scrape_min_interval = 900; // 15 minutes
-  sessionSettings.cache_size = pref.diskCacheSize() ? pref.diskCacheSize() * 64 : -1;
+  int cache_size = pref.diskCacheSize();
+  sessionSettings.cache_size = cache_size ? pref.diskCacheSize() * 64 : -1;
   qDebug() << "Using a disk cache size of" << pref.diskCacheSize() << "MiB";
   // Disable OS cache to avoid memory problems (uTorrent behavior)
 #ifdef Q_WS_WIN