BUG #15262: "unexpected end of tuplestore" error when using new GROUPS window function clause

The following bug has been logged on the website:

Bug reference: 15262
Logged by: Lukas Eder
Email address: lukas(dot)eder(at)gmail(dot)com
PostgreSQL version: 11beta2
Operating system: Debian 6.3.0-18+deb9u1
Description:

This error can be reproduced easily as follows

postgres=# SELECT count(*) OVER (GROUPS 1 PRECEDING) FROM (VALUES (1),
(2), (2)) t(v);
ERROR: unexpected end of tuplestore

Notice the missing ORDER BY clause in the window specification. This works
as expected:

postgres=# SELECT count(*) OVER (ORDER BY v GROUPS 1 PRECEDING) FROM
(VALUES (1), (2), (2)) t(v);
count
-------
1
3
3
(3 rows)

I used the latest 11 beta 2 version on Docker:

postgres=# select version();
version

-----------------------------------------------------------------------------------------------------------------------------------------
PostgreSQL 11beta2 (Debian 11~beta2-1.pgdg90+1) on x86_64-pc-linux-gnu,
compiled by gcc (Debian 6.3.0-18+deb9u1) 6.3.0 20170516, 64-bit
(1 row)