Carefully check records when forcibly invalidating
    
    We should never try to invalidate an already invalid record as
    internal pointers will not be consistent. Carefully test that the
    record really is valid when we are fishing for free space, and
    properly invalidate records or return a fatal error if something
    goes wrong.
    In order to make the code more robust always invalidate the whole
    data space on initialization by setting all bits to 1, and make sure
    to invalidate the whole last allocated slot by converting rec->len to
    the number of slots instead of just the space used.