The problem was that wrong structure of mysql.event was not detected and
the server continued to use wrongly-structured data.

The fix is to check the structure of mysql.event after opening before
any use. That makes operations with events more strict -- some operations
that might work before throw errors now. That seems to be Ok.

Another side-effect of the patch is that if mysql.event is corrupted,
unrelated DROP DATABASE statements issue an SQL warning about inability
to open mysql.event table.