typo
The boundary-next field points to the next boundary structure we were
going to use, but there's no requirement that value be incremented
before reconciliation updates the structure it points to, that is,
there's no guarantee elements of the next boundary structure are still
unchanged. Be defensive, clean up the "next" structure as well as the
ones we know we used.  Reference #1239.

Minor cleanups to clean out the structure more completely.