Forget old filename on starting a new log.

Fixes #253.

Signed-off-by: Robert C. Helling <helling@atdotde.de>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

qt-ui/mainwindow.cpp

@@ -141,6 +141,7 @@ void MainWindow::on_actionClose_triggered()
 	/* clear the selection and the statistics */
 	selected_dive = -1;
 
+	existing_filename = NULL;
 	cleanUpEmpty();
 	mark_divelist_changed(FALSE);