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.

src/test/regress/sql/errors.sql

@@ -37,6 +37,10 @@ select * from pg_database where pg_database.datname = nonesuch;
 -- bad attribute name in select distinct on
 select distinct on (foobar) * from pg_database;
 
+-- grouping with FOR UPDATE
+select null from pg_database group by datname for update;
+select null from pg_database group by grouping sets (()) for update;
+
 
 --
 -- DELETE