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.