Avoid crashing when calling an invalid address.

We already have a check, let's use it properly.
Remove incorrect atrac decode ptr nullcheck.

Already shown in decode test to be valid.
Keep a buffer for atrac to output the right size.

The first chunk is always smaller.  Seems to align to 0x800.
Correct the dataOff member of atrac context.
Add breakpoints to most sceAtrac mem access.
Fix seeking in atrac after the start.

Not sure the very start is right though, arg.
Return the correct next sample at the beginning.
Clamp the final sample count during decode.

Some games depend on / expect this, or else they'll let important data get
overwritten.
Add extra frames if we run out of atrac data.

We could probably insert frames instead for GHA phase shifting, but this
will solve other bugs too, I think.