(BUG#2393)


mysql-test/r/subselect.result:
  BUG#2393 test suite
  test ref replaced ref_of_null joining
mysql-test/t/subselect.test:
  BUG#2393 test suite
  test ref replaced ref_of_null joining
sql/item_subselect.cc:
  avoid null check on fields where NULL is impossible
sql/sql_select.cc:
  convert JT_REF_OR_NULL to JT_REF in case of impossible NULL and protect buffer of rewriting through join_tab->null_ref_key