Database*: fix nullptr dereference when no database is configured

src/DatabaseQueue.cxx

@@ -49,11 +49,15 @@ bool
 findAddIn(struct player_control *pc, const char *uri,
 	  const struct locate_item_list *criteria, GError **error_r)
 {
+	const Database *db = GetDatabase(error_r);
+	if (db == nullptr)
+		return false;
+
 	const DatabaseSelection selection(uri, true, criteria);
 
 	using namespace std::placeholders;
 	const auto f = std::bind(AddToQueue, pc, _1, _2);
-	return GetDatabase()->Visit(selection, f, error_r);
+	return db->Visit(selection, f, error_r);
 }
 
 bool