The previous logic would fail to set groupList when
grouping_is_sortable() returned false, which could cause queries
to return wrong answers when some columns were not sortable.

David Rowley, reviewed by Heikki Linnakangas and Álvaro Herrera.
Reported by Hubert "depesz" Lubaczewski.

Discussion: http://postgr.es/m/Zktzf926vslR35Fv@depesz.com
Discussion: http://postgr.es/m/CAApHDvra=c8_zZT0J-TftByWN2Y+OJfnjNJFg4Dfdi2s+QzmqA@mail.gmail.com