Fix tpt.message_box correctly to only accept true/false third arg.

src/cat/LegacyLuaAPI.cpp

@@ -1739,7 +1739,7 @@ int luatpt_message_box(lua_State* l)
 {
 	std::string title = std::string(luaL_optstring(l, 1, "Title"));
 	std::string message = std::string(luaL_optstring(l, 2, "Message"));
-	int large = luaL_optint(l, 3, 0);
+	int large = lua_isboolean(l, 3)?lua_toboolean(l, 3):0;
 	new InformationMessage(title, message, large);
 	return 0;
 }