r6528: - in tdb_fetch() we effectively disallowed zero length records by

r6528: - in tdb_fetch() we effectively disallowed zero length records by
  returning NULL/0, which is the same as we used for a failure. Having
  to look at tdb->ecode (which we never do) is too error prone.

  Instead, tdb_fetch() should behave like malloc() and talloc(), where
  zero length is not special and malloc(0) returns a valid pointer.

- similarly in data_blob(), asking for data_blob(NULL, 0) should
  return a zero blob, but asking for data_blob(ptr, 0) should return a
  zero length blob with a valid pointer, just like talloc() and malloc()

This change fixes the SummaryInformation stream stored in the tdb
backend when manipulated from w2k. The w2k client was using
SET_EOF_INFORMATION to create a zero-length stream, which we return
STATUS_NOT_FOUND on, as the tdb_fetch() gave us back a NULL/0 blob,
which we returned as not-found