Backport from mysql-5.5 to mysql-5.1

Bug #19612819 :  FILESORT: ASSERTION FAILED: POS->FIELD != 0 || POS->ITEM != 0

Problem:
While getting the temp table field for a REF_ITEM
make_sortorder is using the real_item. As a result
server fails later with an assert.

Solution:
Do not use real_item to get the temp table field.
Instead use the REF_ITEM itself as temp table fields
are created for REF_ITEM not the real_item.