null_ref_key moved to TABLE_REF.
new null range created if necessary.


mysql-test/r/order_by.result:
  fix for the bug #2419: order by ignores rows
mysql-test/t/order_by.test:
  fix for the bug #2419: order by ignores rows
sql/item_subselect.cc:
  fix for the bug #2419: order by ignores rows
sql/opt_range.cc:
  fix for the bug #2419: order by ignores rows
sql/sql_select.cc:
  fix for the bug #2419: order by ignores rows
sql/sql_select.h:
  fix for the bug #2419: order by ignores rows