could: Fix 10788 cloud volume deletion crash

could: Fix 10788 cloud volume deletion crash

- the problem is about iterating up to including parts.last_index()
  using parts.get(parts.last_index()) is a mistake !!!
  it works most of the time because most of the time this value is NULL
  and the code does a continue
  BUT when parts.last_index() == parts.max_size(), then
  parts.get(parts.last_index()) is out of the allocated memory and can
  contain garbage
- why is the code doing a "continue" ? Is it expected to have a NULL ?
  in the list ?
  in clean_cloud_volume_read_cb() that fill the list, there is no raison
  to have NULL ?
  With the bug, testing for NULL will save us 99% of the time
  Without the bug we can reach 100% :-)