Explain fails at fix_fields stage and some items are left unfixed,
particulary Item_group_concat. Item_group_concat::orig_args field
is uninitialized in this case and Item_group_concat::print call 
leads to crash.
The fix:
move the initialization of Item_group_concat::orig_args
into constructor.


mysql-test/r/func_gconcat.result:
  test case
mysql-test/t/func_gconcat.test:
  test case
sql/item_sum.cc:
  move the initialization of Item_group_concat::orig_args
  into constructor.