Fix pfree issue in presorted DISTINCT aggregate code

Fix pfree issue in presorted DISTINCT aggregate code

The logic in this area was recently changed in 7da51590e, however, in that
commit, I neglected to consider that the conditions in which we should
pfree the old Datum needed to be updated after that change.  This could
result in trying to pfree a NULL value, as was demonstrated by Alexander
Lakhin.

Reported-by: Alexander Lakhin
Discussion: https://postgr.es/m/4103db46-d888-6d1d-e88d-87c21ed99472@gmail.com