- handle crashes where the table definition has been changed
  (different number of fields)


sql/event.cc:
  change the way table is opened
  check whether the number of fields is exact, otherwise report damaged table
sql/event_executor.cc:
  - move around some code
  - use the new way of table opening
sql/event_priv.h:
  - new declaration
sql/event_timed.cc:
  - now 0 is ok