Memory allocated in TMP_TABLE_PARAM::copy_field is not cleaned up.
The fix is to clean up TMP_TABLE_PARAM::copy_field array in JOIN::destroy.


mysql-test/r/explain.result:
  test result
mysql-test/t/explain.test:
  test case
sql/sql_select.cc:
  Memory allocated in TMP_TABLE_PARAM::copy_field is not cleaned up.
  The fix is to clean up TMP_TABLE_PARAM::copy_field array in JOIN::destroy.