Fix [modify_unit] optimized path producing an error on a recall list unit

Addresses #4556

data/lua/wml/modify_unit.lua

@@ -182,7 +182,8 @@ local function simple_modify_unit(cfg)
 
 		-- by default always do an advancement here (not only when experience/max_experience/type was modified)
 		-- for compatibility with old code.
-		if cfg.experience or cfg.max_experience or cfg.rebuild ~= false then
+		-- Skip for recall list units
+		if u.valid == 'map' and (cfg.experience or cfg.max_experience or cfg.rebuild ~= false) then
 			u:advance()
 		end
 	end

src/scripting/game_lua_kernel.cpp

@@ -3276,7 +3276,6 @@ static int intf_remove_modifications(lua_State *L)
 static int intf_advance_unit(lua_State *L)
 {
 	events::command_disabler command_disabler;
-	//TODO: check whether the unit is on the map.
 	unit& u = luaW_checkunit(L, 1, true);
 	advance_unit_params par(u.get_location());
 	if(lua_isboolean(L, 2)) {