Changes in an item tree done by optimizer weren't properly
registered and went unnoticed, which resulted in preliminary freeing
of used memory.


mysql-test/r/ps.result:
  Add result for bug#21166: Prepared statement causes signal 11
  on second execution.
mysql-test/t/ps.test:
  Add test case for bug#21166: Prepared statement causes signal 11
  on second execution.
sql/item.cc:
  Move Item::transform() and Item_default_value::transform() from
  item.h here and use THD::change_item_tree() instead of plain
  assignment.
  Change Item_field::set_no_const_sub() to be used with Item::walk()
  instead of Item::transform().
sql/item.h:
  Move definition of Item::transform() and Item_default_value::transform()
  to item.cc.
  Change Item::set_no_const_sub() to be used with Item::walk()
  instead of Item::transform().
sql/item_cmpfunc.cc:
  Use Item::walk() to execute Item::set_no_const_sub().
  Use THD::change_item_tree() instead of plain assignment.
sql/item_func.cc:
  Add assert and comment to Item_func::traverse_cond().
sql/item_row.cc:
  Use THD::change_item_tree() instead of plain assignment.
sql/item_strfunc.cc:
  Move Item_func_make_set::transform() from item_strfunc.h here and use
  THD::change_item_tree() instead of plain assignment.
sql/item_strfunc.h:
  Move definition of Item_func_make_set::transform() to item_strfunc.cc.