Implement handler for cancel button of Save As dialog

When an user opened the "Save as" dialog and pressed the
cancel button a null string was returned. Therefore the
file_save_as function returned an error which was lately
shown when the file_save function was called.
Now the function checks if the cancel/exit button was
pressed and returns.

Fixes #844

Reported-by: longjohnsilver
Signed-off-by: Claudiu Olteanu <olteanu.claudiu@ymail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

qt-ui/mainwindow.cpp

@@ -1259,8 +1259,15 @@ int MainWindow::file_save_as(void)
 {
 	QString filename;
 	const char *default_filename = existing_filename;
-	filename = QFileDialog::getSaveFileName(this, tr("Save file as"), default_filename,
+	QFileDialog selection_dialog(this, tr("Save file as"), default_filename,
 	                             tr("Subsurface XML files (*.ssrf *.xml *.XML)"));
+
+	/* if the exit/cancel button is pressed return */
+	if (!selection_dialog.exec())
+		return 0;
+
+	/* get the first selected file */
+	filename = selection_dialog.selectedFiles().at(0);
 	if (filename.isNull() || filename.isEmpty())
 		return report_error("No filename to save into");