Fixed a bug in the rewrite cache (see :doc set-rw-cache-state).

Quoting :doc note-7-0:

  Fixed a bug that was causing a low-level assertion, saying:
  ``Implementation error: see 'replace-free-rw-cache-entry.'' Thanks
  to Anna Slobodova for bringing this bug to our attention via an
  example, and to Sol Swords for simplifying that example.