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.