Test case:

CREATE TABLE T1 ( F1 SMALLINT );

CREATE PROCEDURE P1
AS
  DECLARE VARIABLE V1 SMALLINT;
  DECLARE C1 CURSOR FOR ( SELECT F1 FROM T1 );
BEGIN
  OPEN C1;
  WHILE (1=1) DO
  BEGIN
       FETCH C1 INTO :V1;
       IF(ROW_COUNT = 1) THEN LEAVE;
  END
  CLOSE C1;
END;

COMMIT;

EXECUTE PROCEDURE P1;

Unsuccessful execution caused by a system error that precludes
successful execution of subsequent statements.
Internal gds software consistency check (pointer page vanished from DPM_next (249), file: dpm.cpp line: 1645).

While this example looks like an intentional infinite loop, I believe we should mirror our API behaviour for such situations and throw an error for the invalid (outside the EOF marker) fetch operation.