Fixed provisional-certification bug for verify-guards+ forms and, more generally, make-event forms with :expansion? supplied.

Quoting :doc note-7-0:

  Fixed a bug in [provisional-certification] that incorrectly caused an
  error during the convert step for [verify-guards+] forms. More
  generally, this error occurred when encountering a [make-event]
  form with a non-[nil] value supplied for the :expansion? keyword.
  Thanks to David Rager for reporting this bug.