fix 'at_end()' assertion faliure when chatting in a mp game

See #1857

This commit does 2 things:
1) Ensure that if the replay pos was at_end before a speak was added it will still be at end after the speak was added, fixes the mentioned assertion failure.
2) Make unsyced map_labels use the same logic as speak, this fixes OOS issues when the 'back to turn' mp feature was used in mp, since previously adding map labels while watching a mp replay (is the really possible? not 100% sure) might cause mismatches of replay_pos of other savegames with the current replay data.

src/replay.cpp

@@ -337,8 +337,7 @@ bool replay::add_chat_message_location(int pos)
 
 void replay::speak(const config& cfg)
 {
-	config& cmd = base_->insert_command(base_->size());
-	cmd["undo"] = false;
+	config& cmd = add_nonundoable_command();
 	cmd.add_child("speak",cfg);
 	add_chat_message_location(base_->size() - 1);
 }
@@ -581,9 +580,13 @@ config& replay::add_command()
 
 config& replay::add_nonundoable_command()
 {
-	config& r = base_->insert_command(base_->get_pos());
+	const bool was_at_end = at_end();
+	config& r = base_->insert_command(base_->size());
 	r["undo"] = false;
-	base_->set_pos(base_->get_pos() + 1);
+	if(was_at_end) {
+		base_->set_pos(base_->get_pos() + 1);
+	}
+	assert(was_at_end == at_end());
 	return r;
 }