Don't cache per-group context across the whole query in orderedsetaggs.c.

Although nodeAgg.c currently uses the same per-group memory context for
all groups of a query, that might change in future.  Avoid assuming it.
This costs us an extra AggCheckCallContext() call per group, but that's
pretty cheap and is probably good from a safety standpoint anyway.

Back-patch to 9.4 in case any third-party code copies this logic.

Andrew Gierth