BUG #11130: Case condition error

The following bug has been logged on the website:

Bug reference:      11130
Logged by:          Laszlo Mozsar
Email address:      mozsar.laszlo@mgmail.com
PostgreSQL version: 9.3.4
Operating system:   Ubuntu 14.04 LTS
Description:

The "is null" contition on a date field returns wrong result. I can't
reproduce this with a test table. My tables contains personal and business
informations, so I can't send a full log with datas.
(I used reindex and vacuum before submitting this report.)

The structure:
CREATE TABLE public.hitel (
  kod INTEGER NOT NULL,
  szerzodes_szam VARCHAR(40) NOT NULL,
  szerzodes_datum DATE NOT NULL,
  lejarat_datum DATE,
  lejarat_tipus CHAR(1),
  szerzodeskod_merkint VARCHAR(20),
  szerzodeskod_bag VARCHAR(20),
  szerzodeskod_elementa VARCHAR(20),
  szerzodeskod_eurobank VARCHAR(40),
  ugyfelkod_merkint VARCHAR(20),
  ugyfelkod_bag VARCHAR(20),
  ugyfelkod_elementa VARCHAR(20),
  ugyfelkod_eurobank VARCHAR(20),
  szamlaszam public.szamlaszam,
  folyoszamlaszam public.szamlaszam,
  deviza public.deviza,
  indulo_toke public.osszeg NOT NULL,
  kulonbozet public.osszeg,
  elszamolas_modja CHAR(1),
  tamogatott BOOLEAN NOT NULL,
  folyoszamla BOOLEAN NOT NULL,
  bapo BOOLEAN NOT NULL,
  konvertalt_errol INTEGER,
  konvertalt_erre INTEGER,
  hirdetmeny INTEGER,
  hirdetmeny_forras VARCHAR(20),
  szerzodeskod_flex VARCHAR(20),
  ugyfelkod_flex VARCHAR(20),
  ugyfelnev VARCHAR(80),
  koveteles_huf public.osszeg,
  koveteles public.osszeg,
  CONSTRAINT hitel_pkey PRIMARY KEY(kod),
  CONSTRAINT hitel_fk_hirdetmeny FOREIGN KEY (hirdetmeny)
    REFERENCES public.hirdetmeny(kod)
    ON DELETE NO ACTION
    ON UPDATE NO ACTION
    NOT DEFERRABLE,
  CONSTRAINT hitel_fk_konvertalt_erre FOREIGN KEY (konvertalt_erre)
    REFERENCES public.hitel(kod)
    ON DELETE NO ACTION
    ON UPDATE NO ACTION
    NOT DEFERRABLE,
  CONSTRAINT hitel_fk_konvertalt_errol FOREIGN KEY (konvertalt_errol)
    REFERENCES public.hitel(kod)
    ON DELETE NO ACTION
    ON UPDATE NO ACTION
    NOT DEFERRABLE
)
WITH (oids = false);

The SQL select:
select
    h.szerzodes_szam,
    case
      when h.lejarat_datum is null and h.lejarat_tipus is null then 'Elo'
      when h.lejarat_tipus = 'L' then 'Felmondott'
      else 'Lezart'
    end as statusz,
    h.lejarat_datum,
    h.lejarat_tipus,
    h.deviza,
    coalesce (h.koveteles, 0.00) as koveteles,
    coalesce (h.koveteles_huf, 0.00) as koveteles_huf,
    'E' as jelleg
  from hitel h

The result contains many rows with statusz='Lezart', but lejarat_tipus and
lejarat_datum are nulls. The result not contains any record with
statusz='Elo' and lejarat_datum is not null.

If I use this, the result is correct:
      when coalesce (h.lejarat_datum, '1899-12-30') = '1899-12-30' and
h.lejarat_tipus is null then 'Elo'

PS: Sorry, my engllish is poor. :(