MDEV-17070 Table corruption or Assertion `table->file->stats.records > 0 || error' or Assertion `!is_set() || (m_status == DA_OK_BULK && is_bulk_op())' failed upon actions on temporary table

This was caused by a combination of factors:
* MyISAM/Aria temporary tables historically never saved the state
  to disk (MYI/MAI), because the state never needed to persist
* certain ALTER TABLE operations modify the original TABLE structure
  and if they fail, the original table has to be reopened to
  revert all changes (m_needs_reopen=1)

as a result, when ALTER fails and MyISAM/Aria temp table gets reopened,
it reads the stale state from the disk.

As a fix, MyISAM/Aria tables now *always* write the state to disk
on close, *unless* HA_EXTRA_PREPARE_FOR_DROP was done first. And
the server now always does HA_EXTRA_PREPARE_FOR_DROP before dropping
a temporary table.