Bug#1091519: deb-systemd-helper: Fails when non-existing unit is listed in Also= line

[Daniel Hornung]

This bug hits users like me, when following the Bookworm -> Trixie update 
instructions on

> apt purge '?config-files'

[...]
Purging configuration files for libvirt-daemon-system (9.0.0-4+deb12u2) ...
Deep recursion on subroutine "main::remove_links" at /usr/bin/deb-systemd-
helper line 488, <$fh> line 14.
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