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

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