Problem: EXPLAIN EXTENDED was trying to resolve references to 
freed temporary table fields for GROUP_CONCAT()'s ORDER BY arguments.

Fix: use stored original GROUP_CONCAT()'s arguments in such a case.


mysql-test/r/func_gconcat.result:
  Fix for bug#52397: another crash with explain extended and group_concat
    - test result.
mysql-test/t/func_gconcat.test:
  Fix for bug#52397: another crash with explain extended and group_concat
    - test case.
sql/item_sum.cc:
  Fix for bug#52397: another crash with explain extended and group_concat
    - use "pargs", printing ORDER BY arguments in the 
  Item_func_group_concat::print() instead of "order" to avoid
  possible reference resolving to (freed) temporary table fields.