mtr_t::commit_shrink(): Do not assert that some previously clean pages
will be flagged as modified by this mini-transaction. It could be the
case that there had been no recent write-back of any of the undo
tablespace pages that we are modifying when truncating the tablespace.
It suffices to assert that some pages were modified again:
ut_ad(m_modifications).

This fixes up commit f5fddae3