Fix error: control may reach end of non-void lambda

If by_name and by_relation are both omitted, behave as if both were given: sort first by relation then by name.

src/game_initialization/lobby_info.cpp

@@ -405,6 +405,8 @@ void lobby_info::sort_users(bool by_name, bool by_relation)
 		if(by_relation) {
 			return u1->relation < u2->relation;
 		}
+
+		return true;
 	});
 }