cursor already in use, UPDATE RETURNING bug?

Hi,

Just noted an interesting behaviour when using a cursor in a function
in an UPDATE RETURNING (note that INSERT RETURNING has no problem).

I have seen this problem in all versions I tested (9.4 thru master).
Steps to reproduce:

prepare the test
```
create table t1 as select random() * foo i from generate_series(1, 100) foo;
create table t2 as select random() * foo i from generate_series(1, 100) foo;

CREATE OR REPLACE FUNCTION cursor_bug()
 RETURNS integer
 LANGUAGE plpgsql
AS $function$
declare
  c1 cursor (p1 int) for select count(*) from t1 where i = p1;
  n int4;
begin
  open c1 (77);
  fetch c1 into n;
  return n;
end $function$
;
```

-- this ends fine
insert into t2 values(5) returning cursor_bug() as c1;
 c1
----
  0
(1 row)

-- this fails
update t2 set i = 5 returning cursor_bug() as c1;
ERROR:  cursor "c1" already in use
CONTEXT:  PL/pgSQL function cursor_bug() line 6 at OPEN

-- 
Jaime Casanova
Director de Servicios Profesionales
SYSTEMGUARDS - Consultores de PostgreSQL