Calling List<Cached_item>::delete_elements for the same list twice
caused a crash of the server in the function JOIN::cleaunup.
Ensured that delete_elements() in JOIN::cleanup would be called only once.


mysql-test/r/subselect.result:
  Added a test case for bug #38191.
mysql-test/t/subselect.test:
  Added a test case for bug #38191.
sql/sql_select.cc:
  Fixed bug #38191.
  Ensured that delete_elements() in JOIN::cleanup would be called only once.