Cleanup: Use std::advance instead of for-loop.

src/landscape.cpp

@@ -1378,11 +1378,9 @@ static std::tuple<bool, bool> FlowRiver(TileIndex spring, TileIndex begin, uint
 		std::tie(found, main_river) = FlowRiver(spring, end, min_river_length);
 	} else if (count > 32) {
 		/* Maybe we can make a lake. Find the Nth of the considered tiles. */
-		TileIndex lakeCenter = 0;
-		int i = RandomRange(count - 1) + 1;
-		std::set<TileIndex>::const_iterator cit = marks.begin();
-		while (--i) cit++;
-		lakeCenter = *cit;
+		std::set<TileIndex>::const_iterator cit = marks.cbegin();
+		std::advance(cit, RandomRange(count - 1));
+		TileIndex lakeCenter = *cit;
 
 		if (IsValidTile(lakeCenter) &&
 				/* A river, or lake, can only be built on flat slopes. */

src/script/script_gui.cpp

@@ -174,9 +174,9 @@ struct ScriptListWindow : public Window {
 		if (this->selected == -1) {
 			GetConfig(slot)->Change(nullptr);
 		} else {
-			ScriptInfoList::const_iterator it = this->info_list->begin();
-			for (int i = 0; i < this->selected; i++) it++;
-			GetConfig(slot)->Change((*it).second->GetName(), (*it).second->GetVersion());
+			ScriptInfoList::const_iterator it = this->info_list->cbegin();
+			std::advance(it, this->selected);
+			GetConfig(slot)->Change(it->second->GetName(), it->second->GetVersion());
 		}
 		InvalidateWindowData(WC_GAME_OPTIONS, slot == OWNER_DEITY ? WN_GAME_OPTIONS_GS : WN_GAME_OPTIONS_AI);
 		InvalidateWindowClassesData(WC_SCRIPT_SETTINGS);