pgsql: Ensure result of an aggregate's finalfunc is made read-only.

Ensure result of an aggregate's finalfunc is made read-only.

The finalfunc might return a read-write expanded object.  If we
de-duplicate multiple call sites for the aggregate, any function(s)
receiving the aggregate result earlier could alter or destroy the
value that reaches the ones called later.  This is a brown-paper-bag
bug in commit 42b746d4c, because we actually considered the need
for read-only-ness but failed to realize that it applied to the case
with a finalfunc as well as the case without.

Per report from Justin Pryzby.  New error in HEAD,
no need for back-patch.

Discussion: https://postgr.es/m/ZDm5TuKsh3tzoEjz@telsasoft.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/78d5952dd0e66afc4447eec07f770991fa406cce

Modified Files
--------------
src/backend/executor/nodeAgg.c           | 18 +++++++++++----
src/backend/executor/nodeWindowAgg.c     |  7 +++++-
src/test/regress/expected/aggregates.out | 37 ++++++++++++++++++++++++++++++
src/test/regress/sql/aggregates.sql      | 39 ++++++++++++++++++++++++++++++++
4 files changed, 96 insertions(+), 5 deletions(-)