pgsql: Fix partial aggregation for variance(int4) and relatedaggregate

Fix partial aggregation for variance(int4) and related aggregates.

A typo in numeric_poly_combine caused bogus results for queries using
it, but of course would only manifest if parallel aggregation is
performed.  Reported by Rajkumar Raghuwanshi.

David Rowley did the diagnosis and the fix; I editorialized rather
heavily on his regression test additions.

Back-patch to v10 where the breakage was introduced (by 9cca11c91).

Discussion: https://postgr.es/m/CAKcux6nU4E2x8nkSBpLOT2DPvQ5LviJ3SGyAN6Sz7qDH4G4+Pw@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/ec4719cd155d1d58c8aa7c06c7ef24aef6e67622

Modified Files
--------------
src/backend/utils/adt/numeric.c          |  4 ++--
src/test/regress/expected/aggregates.out | 27 +++++++++++++++++++++++++++
src/test/regress/sql/aggregates.sql      | 18 ++++++++++++++++++
3 files changed, 47 insertions(+), 2 deletions(-)