[Merge] ~powersj/livecd-rootfs/+git/livecd-rootfs:fix-unbound-variable into livecd-rootfs:ubuntu/master

[Michael Hudson-Doyle]

Review: Approve

This is fine and I'll merge it but I will note that only live-build/ubuntu-core/hooks/00-uid-gid-fix.chroot_early and live-build/ubuntu-core/hooks/99zz-check-uid-gid.chroot in the livecd-rootfs tree run with set -u in effect. I'm guessing this is hit in a CPC hook? In any case, we may want to be more consistent here.
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https://code.launchpad.net/~powersj/livecd-rootfs/+git/livecd-rootfs/+merge/387329
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