MP3File: tweak buffer handling
MP3File: tweak buffer handling

After adding the ability to change files in an existing MP3File object,
it became apparent that at the beginning of a track some part of an
existing buffer was playing first.

I noticed that in get_buffer, the just-populated buffer wasn't being
returned, but the other one was.  But still after fixing this, I heard
wrong audio at the beginning of a track, so I took the heavy duty approach
and zeroed the buffers out.  That means there's a remaining bug to chase,
which is merely hidden by the memset()s.

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