Comment 15 for bug 2058648

https://salsa.debian.org/helmutg/bootstrap-usrmerge-demo/-/commit/49a35aa03472277b80406e152ee96989d8081d45 is my change for the Debian side (which has not reached unstable yet) and adds a safe-guard to libc6.preinst. I think that would be reasonable to include in Ubuntu too.