fix possible assertion failure in mp when a player is leaving.

see the similar 1.13 commit 23fdeba
see the report in the forum: https://forums.wesnoth.org/viewtopic.php?f=4&p=596854

src/playsingle_controller.cpp

@@ -927,6 +927,7 @@ void playsingle_controller::play_ai_turn(){
 		// to record this change for the replay though.
 		synced_context::run_in_synced_context("auto_shroud", replay_helper::get_auto_shroud(true));
 	}
+	undo_stack_->clear();
 
 	turn_info turn_data(player_number_, replay_sender_, network_reader_);