CHECK OPTION and a subquery in WHERE condition.
The abort was triggered by setting the value of join->tables for
subqueries in the function JOIN::cleanup. This function was called
after an invocation of the JOIN::join_free method for subqueries
used in WHERE condition.


mysql-test/r/view.result:
  Added a test case for bug #28561.
mysql-test/t/view.test:
  Added a test case for bug #28561.
sql/sql_select.cc:
  Fixed bug #28561: assertion abort for update on multi-table view with
  CHECK OPTION and a subquery in WHERE condition.
  The abort was triggered by setting the value of join->tables for
  subqueries in the function JOIN::cleanup. This function was called
  after an invocation of the JOIN::join_free method for subqueries
  used in WHERE condition.
  Setting the value of join->tables to for a subquery created serious
  problems for checking WHERE condition after update of the multi-table
  view as this check is performed in the do_select function right
  after a call of the JOIN::join_free method.
  In fact setting join->tables to 0 in JOIN::cleanup is not needed 
  anywhere in the current code.