Clear old tree counter in map array on load of old savegames

See: https://github.com/OpenTTD/OpenTTD/pull/10018

src/saveload/afterload.cpp

@@ -4098,6 +4098,14 @@ bool AfterLoadGame()
 		}
 	}
 
+	if (SlXvIsFeatureMissing(XSLFI_NO_TREE_COUNTER)) {
+		for (TileIndex t = 0; t < map_size; t++) {
+			if (IsTileType(t, MP_TREES)) {
+				ClearOldTreeCounter(t);
+			}
+		}
+	}
+
 	InitializeRoadGUI();
 
 	/* This needs to be done after conversion. */