When making a place to store field values at the start of each group
  the real item (not the reference) must be used when deciding which column
  to copy.


mysql-test/r/group_by.result:
  Bug #20466: a view is mixing data when there's a trigger on the table
   - test suite for the bug
mysql-test/t/group_by.test:
  Bug #20466: a view is mixing data when there's a trigger on the table
   - test suite for the bug
sql/sql_select.cc:
  Bug #20466: a view is mixing data when there's a trigger on the table
   - deal correctly with references