The optimizer transforms DISTINCT into a GROUP BY
when possible.
It does that by constructing the same structure
(a list of ORDER instances) the parser makes when
parsing GROUP BY.
While doing that it also eliminates duplicates.
But if a duplicate is found it doesn't advance the
pointer to ref_pointer array, so the next 
(and subsequent) ORDER structures point to the wrong
element in the SELECT list.
Fixed by advancing the pointer in ref_pointer_array
even in the case of a duplicate.