Fix a double-free in load_file_object's error handling

[LionsPhil]

unload_datafile() already frees the datafile itself, so calling _AL_FREE() on it again is a double-free. This causes custom data loaders returning errors to abruptly abort the program before any error message from them can reach output (or worse just corrupt the heap if it's not caught).

[LionsPhil]

This bug is in upstream Allegro, but I believe https://github.com/liballeg/allegro4 is just intended as a historical archive from their news post: "Note that Allegro 4 should still be treated as mostly abandoned, and won’t be receving regular updates. This change is for the convenience of the few remaining users of it, and to enable easier historical exploration of the repository using Github’s tooling."

Note my debugging for it is done against upstream. I haven't done a build/test here, but it's a simple spot-fix where you can trace the problematic free across a single call.