Fix crash when adding torrent via a magnet url

The file guard object has to be initialised in this case too because it
is accessed when dialog gets accepted or cancelled. Initialise it with
empty path.

src/gui/addnewtorrentdialog.cpp

@@ -259,6 +259,7 @@ bool AddNewTorrentDialog::loadMagnet(const BitTorrent::MagnetUri &magnetUri)
         return false;
     }
 
+    m_torrentGuard.reset(new TorrentFileGuard(QString()));
     m_hash = magnetUri.hash();
     // Prevent showing the dialog if download is already present
     if (BitTorrent::Session::instance()->isKnownTorrent(m_hash)) {