Fix prewrite callbacks' cargo list update for in-place update

Fix prewrite callbacks' cargo list update for in-place update

When updating in-place (old==new), we must not write through new
before we're done reading the same memory through old.

Bug: we write the carrier uids too early.  Cargo lists aren't updated
when a carrier dies in an in-place update.  No such updates are known.
Broken in commit 64a53c90, v4.3.17.