Fixed bug #12095: a join query with GROUP_CONCAT over a single row table.

If GROUP_CONCAT is calculated we always put its argument into a temp
table, even when the argument is a constant item.

sql/item_sum.cc

@@ -1986,6 +1986,7 @@ bool Item_func_group_concat::setup(THD *thd)
   }
   
   count_field_types(tmp_table_param,all_fields,0);
+  tmp_table_param->need_const= 1;
   if (table)
   {
     /*