Procedure - Retorna último dia do mês

SQL: 

create or alter procedure ULTIMODIAMES (MES integer, ANO integer) returns (ULTIMO_DIA smallint) as
  declare variable VPROXMES integer;
  declare variable DATA date;
begin
  if (:mes is null) then
    mes=EXTRACT(month FROM CURRENT_DATE);

  if (:ano is null) then
    ano=EXTRACT(YEAR FROM CURRENT_DATE);

  vproxmes = :mes+1;

  if (:mes=12) then
  begin
    ano= :ano+1;
    vproxmes = 1;
  end

  DATA = '01.'||:vproxmes||'.'||ANO;
  ultimo_dia = EXTRACT(DAY FROM (CAST(DATA AS DATE)-1));
  suspend;
end

Chamada da procedure: 

SELECT * 
  FROM ULTIMODIAMES(07, 2019)

Retorno:

31