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

bin/varnishd/cache/cache_ban_lurker.c

@@ -145,9 +145,6 @@ ban_lurker_test_ban(struct worker *wrk, struct vsl_log *vsl, struct ban *bt,
 	unsigned tests;
 	int i;
 
-	/* It's an error to give an empty list to test against */
-	AZ(VTAILQ_EMPTY(obans));
-
 	/*
 	 * First see if there is anything to do, and if so, insert marker
 	 */