Assert failure with MERGE into partitioned table with RLS

As part of the MERGE RETURNING patch I noticed a suspicious Assert()
in ExecInitPartitionInfo() that looked like it needed updating for
MERGE.

After more testing, I can confirm that this is indeed a pre-existing
bug, that can be triggered using MERGE into a partitioned table that
has RLS enabled (and hence non-empty withCheckOptionLists to
initialise).

So I think we need something like the attached.

Regards,
Dean