Backport from mysql-5.5 to mysql-5.1

Bug#19880368 : GROUP_CONCAT CRASHES AFTER DUMP_LEAF_KEY

Problem:
find_order_by_list does not update the address of order_item
correctly after resolving.

Solution:
Change the ref_by address for a order_by field if its
SUM_FUNC_ITEM to the address of the field present in
all_fields.