ban lurker: the olist can be empty when all bans got dup'ed
ban lurker: the olist can be empty when all bans got dup'ed

All bans on olist can get marked completed as duplicates while we're
working on them, in particular as we start at an offset of
ban_lurker_age.

Fixes #1919

feld/Varnish-Cache,feld/Varnish-Cache,feld/Varnish-Cache,gquintard/Varnish-Cache,feld/Varnish-Cache,feld/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache