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

src/backend/executor/execExprInterp.c

@@ -4254,7 +4254,8 @@ ExecEvalPreOrderedDistinctSingle(AggState *aggstate, AggStatePerTrans pertrans)
 													pertrans->aggCollation,
 													pertrans->lastdatum, value))))
 	{
-		if (pertrans->haslast && !pertrans->inputtypeByVal)
+		if (pertrans->haslast && !pertrans->inputtypeByVal &&
+			!pertrans->lastisnull)
 			pfree(DatumGetPointer(pertrans->lastdatum));
 
 		pertrans->haslast = true;