BUG #17832: ERROR: failed to apply nullingrels to a non-Var in HEAD

The following bug has been logged on the website:

Bug reference:      17832
Logged by:          Marko Tiikkaja
Email address:      marko@joh.to
PostgreSQL version: Unsupported/Unknown
Operating system:   Linux
Description:        

Running the following query in HEAD fails with the error in $SUBJECT:

SELECT 1
FROM
(
    SELECT NULL::text AS AccountName
) Actual FULL JOIN (
    SELECT text 'foo'
    UNION ALL
    SELECT text 'bar'
) Expected (AccountName)
ON TRUE
WHERE Expected.AccountName IS NULL;

I tried PGv15 online and it seemed to work there, so I'm guessing this only
applies to HEAD.