Certain updates of table joined to self results in unexpected
behavior.

The problem was that record cache was mistakenly enabled for
self-joined table updates. Normally record cache must be disabled
for such updates.

Fixed wrong condition in code that determines whether to use
record cache for self-joined table updates.

Only MyISAM tables were affected.


mysql-test/r/myisam.result:
  Test case for BUG#18036.
mysql-test/t/myisam.test:
  Test case for BUG#18036.
sql/sql_update.cc:
  Fixed wrong condition in code that determines whether to use
  record cache for self-joined table updates.