pgsql: Repair crash with unsortable data in grouping sets.

Repair crash with unsortable data in grouping sets.

Previously the code would generate incorrect results, assertion
failures, or crashes if given unsortable (but hashable) columns in
grouping sets. Handle by throwing an error instead.

Report and patch by Pavan Deolasee (though I changed the error
wording slightly); regression test by me.

(This affects 9.5 only since the planner was refactored in 9.6.)

Branch
------
REL9_5_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/7be3678a8cfb55dcfca90fa586485f835ab912a5

Modified Files
--------------
src/backend/optimizer/plan/planner.c | 5 +++++
src/test/regress/expected/groupingsets.out | 7 +++++++
src/test/regress/sql/groupingsets.sql | 4 ++++
3 files changed, 16 insertions(+)