Fix undefined variable in buildSortingSQL

This method was generating invalid SQL because undefined variable $sort_field in line 410 was creating and empty table alias for page_id field. Temporarily replaced $sort_field it with t$field_id expression so that it generates valid SQL. 

I'm afraid this fix isn't definitive because it does not take into account the possiblity that the buildDSRetrievalSQL method was called with $andOperation = true, which would generate different table aliases. Since $andOperation is not available in buildSortingSQL, I'm not sure how to take that into account.