Fix XACT loop loading

For simple sounds (0x02) we don't come across any problems, but for looped/complex sounds (0x03, in our case) the parser will explode when this offset value is wrong.

This aligns properly both for simple sounds (typically the data length is 7, 5 bytes between the extradatalen value and the next sound's flag) and complex sounds (read: it doesn't crash anymore for complex audio). Basically, this extradatalen probably includes the length of the extradatalen value itself.

Test cases appreciated, both functioning and non-functioning.