Fix possible crash when deleting a torrent just after pausing it

src/bittorrent.cpp

@@ -2002,8 +2002,10 @@ void Bittorrent::addConsoleMessage(QString msg, QString) {
       }
 #endif
       else if (torrent_paused_alert* p = dynamic_cast<torrent_paused_alert*>(a.get())) {
+        if(p->handle.is_valid()) {
           p->handle.save_resume_data();
         }
+      }
       else if (tracker_error_alert* p = dynamic_cast<tracker_error_alert*>(a.get())) {
         // Level: fatal
         QTorrentHandle h(p->handle);