Indexed constraint comments not working

Hi all,

Comments on indexed constraints (primary keys, exclusion constraints
etc.) are failing due to referencing the wrong alias of the relevant
pg_description join in the join clause.

Test case:

CREATE TABLE test (id int);

ALTER TABLE test
ADD CONSTRAINT cnt_x_test
EXCLUDE USING btree (id WITH =);

COMMENT ON CONSTRAINT cnt_x_test ON
COMMENT ON CONSTRAINT cnt_x_test
  ON test IS 'Hello';

Viewing the property list of the constraint, the SQL pane and the
constraint editor dialogue doesn't show the comment.

Fix attached.

--
Thom