AGOS: Fix sound offset table access for StS 2 Mac/Amiga.

This (hopefully) fixes bug #6549: "#6549 AGOS: Simon2 Amiga Datafiles crashes with assertion in Intro".
I don't have any copy of StS 2 Mac/Amiga thus I cannot test this.

This bug was caused by a regression in c82a75df69aa5d8f36eae52deee508ef9a61e49e.
Merge pull request #446 from lordhoto/agos-offset-fix

AGOS: Fix sound offset table access for StS 2 Mac/Amiga.