When inserting into a join-based view the update fields from the ON DUPLICATE
KEY UPDATE wasn't checked to be from the table being inserted into and were
silently ignored.

The new check_view_single_update() function is added to check that
insert/update fields are being from the same single table of the view.


sql/sql_insert.cc:
  Bug#25123: ON DUPLICATE KEY clause allows fields not from the insert table.
  The new check_view_single_update() function is added to check that
  insert/update fields are being from the same single table of the view.
mysql-test/r/insert.result:
  Added a test case for bug#25123: ON DUPLICATE KEY clause allows fields not from the insert table.
mysql-test/t/insert.test:
  Added a test case for bug#25123: ON DUPLICATE KEY clause allows fields not from the insert table.