pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ec5a010ab25bf40012fde78b34724971e38e08a4

Modified Files
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src/backend/executor/execExprInterp.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)