Fix seeking past DSP/RPC tables for complex sounds.

This is a renewal of my original XactSound pull request, but for the new develop branch.

For complex sounds (!= 0x02) we currently seek too far, and eventually you'll get a bizarrely huge value that'll cause us to read past the end of the stream.

This fixes the issue, as we're supposed to account for the extraDataLen value itself when seeking past the extra data.

I've got some DSP/RPC table reading in my branch too, but it's a bit ugly at the moment. For now, this will at least fix crashes for anyone that happens to be porting their stuff over.