Fix emptying of piece size combobox when user checks "auto" checkbox.

The function computePathSize returns -1 if file isn't found, not 0.

src/torrentcreator/torrentcreatordlg.cpp

@@ -220,9 +220,9 @@ void TorrentCreatorDlg::on_checkAutoPieceSize_clicked(bool checked)
 
 void TorrentCreatorDlg::updateOptimalPieceSize()
 {
-  quint64 torrent_size = fsutils::computePathSize(textInputPath->text());
+  qint64 torrent_size = fsutils::computePathSize(textInputPath->text());
   qDebug("Torrent size is %lld", torrent_size);
-  if (torrent_size == 0) return;
+  if (torrent_size < 0) return;
   int i = 0;
   qulonglong nb_pieces = 0;
   do {