Fix placeholder access arguments that do not map to objects

[jacobembree]

When a placeholder access argument maps to a loaded object the list of access arguments at position index gets replaced with the loaded object. However, when there's no entity and a specific path component is chosen by number in access_arguments, the number indicating the path component is ignored, only the the index of the number in the list of access arguments is used.

This pull request fixes that by calling arg() with the number specified rather than position of the number.