Cleanup: move clearing of current_dive into clear_dive_file_data()

It makes no sense to have a non-NULL current_dive once all dives
have been deleted. Therefore, clear current_dive implicitly in
clear_dive_file_data() and don't depend on the caller performing
this.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/divelist.c

@@ -1446,6 +1446,7 @@ void clear_dive_file_data()
 {
 	while (dive_table.nr)
 		delete_single_dive(0);
+	current_dive = NULL;
 	while (dive_site_table.nr)
 		delete_dive_site(get_dive_site(0, &dive_site_table), &dive_site_table);
 	if (trip_table.nr != 0) {

desktop-widgets/mainwindow.cpp

@@ -658,7 +658,6 @@ void MainWindow::closeCurrentFile()
 	/* free the dives and trips */
 	clear_git_id();
 	clear_dive_file_data();
-	current_dive = nullptr;
 	setCurrentFile(nullptr);
 	graphics->setEmptyState();
 	mainTab->clearTabs();