Initialize some variables.
Reset decodePos in lowlevel decode.

FFmpeg was smart enough to do this for us, but not when we loaded a
savestate.  This is because without a savestate, we returned an initial
small header packet of this size, which gave FFmpeg the idea that seeking
there might help.

But, since we're forcing the decode at a certain position the better thing
is to seek ourselves.
Merge pull request #6176 from unknownbrackets/atrac-fix

Fix lowlevel atrac looping on savestate load