Was a double-free of the Unique member of Item_func_group_concat.
This was not causing a crash because the Unique is a descendent of
Sql_alloc.
Fixed to free the Unique only if it was allocated for the instance 
of Item_func_group_concat it was referenced from


mysql-test/r/func_gconcat.result:
  Bug #34747: test case
mysql-test/t/func_gconcat.test:
  Bug #34747: test case
sql/item_sum.cc:
  Bug #34747: free the Unique only if it was allocated
  for this instance of Item_func_group_concat