BUG#20769: Dangling pointer in ctype_recoding test case.

In some functions dealing with strings and character sets, the wrong
pointers were saved for restoration in THD::rollback_item_tree_changes().
This could potentially cause random corruption or crashes.

Fixed by passing the original Item ** locations, not local stack copies.

Also remove unnecessary use of default arguments.


sql/item.cc:
  Function agg_item_charsets() now handles non-consequtive Item *'s.
sql/item.h:
  Remove use of default argument.
sql/item_cmpfunc.cc:
  Remove use of default argument.
sql/item_func.cc:
  Remove use of default argument.
sql/item_func.h:
  Function agg_item_charsets() now handles non-consequtive Item *'s.
sql/item_strfunc.cc:
  Pass original Item **'s to agg_arg_charsets(), not local copies, to ensure
  proper restoration in THD::rollback_item_tree_changes().
sql/item_sum.cc:
  Remove use of default argument.

sql/item_sum.cc

@@ -3229,7 +3229,7 @@ Item_func_group_concat::fix_fields(THD *thd, Item **ref)
                         args,
 			/* skip charset aggregation for order columns */
 			arg_count - arg_count_order,
-			MY_COLL_ALLOW_CONV))
+			MY_COLL_ALLOW_CONV, 1))
     return 1;
 
   result.set_charset(collation.collation);