Remove code to use last mp3 handle on error.

It was copied from mpeg, where it was added for safety, and it shouldn't
be hit anyway.
Free mp3 contexts more carefully.
Fix savestate backwards compat.

Broken since 80702109f53968b45d947bdd4495585b68e54220.
Savestate and shutdown sceMp3 properly.

It wasn't even being saved before.
Merge pull request #4308 from unknownbrackets/mp3-minor

Fix mp3 savestates and savestate backwards compat