pgsql: Reject SELECT ... GROUP BY GROUPING SETS (()) FOR UPDATE.

Reject SELECT ... GROUP BY GROUPING SETS (()) FOR UPDATE.

This case should be disallowed, just as FOR UPDATE with a plain
GROUP BY is disallowed; FOR UPDATE only makes sense when each row
of the query result can be identified with a single table row.
However, we missed teaching CheckSelectLocking() to check
groupingSets as well as groupClause, so that it would allow
degenerate grouping sets. That resulted in a bad plan and
a null-pointer dereference in the executor.

Looking around for other instances of the same bug, the only one
I found was in examine_simple_variable(). That'd just lead to
silly estimates, but it should be fixed too.

Per private report from Yaoguang Chen.
Back-patch to all supported branches.

Branch
------
REL9_6_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/3eca18522e1908f7a61ab2545bb9238fce4d1ba7

Modified Files
--------------
src/backend/parser/analyze.c | 2 +-
src/backend/utils/adt/selfuncs.c | 3 ++-
src/test/regress/expected/errors.out | 5 +++++
src/test/regress/sql/errors.sql | 4 ++++
4 files changed, 12 insertions(+), 2 deletions(-)