Remove TMP_TABLE_PARAM::copy_funcs_it. TMP_TABLE_PARAM is a member of JOIN which is 
copied via memcpy, and List_iterator_fast TMP_TABLE_PARAM::copy_funcs_it ends up 
pointing to the wrong List.


mysql-test/r/subselect.result:
  Testcase for BUG#8218
mysql-test/t/subselect.test:
  Testcase for BUG#8218
sql/sql_select.cc:
  Fix for BUG#8218: Create/use own iterator since TMP_TABLE_PARAM::copy_funcs_it is removed.