fellow_cache: fix an lru edge case exposed by fellow_b00062.vtc

For an fco which failed mutation, any fcs of the same fco following
it would be skipped because of

		if (fcs->fco == fco)
			continue;

this was particularly relevant for b62.vtc, which creates just one
cache object.

With the fix, varnishtest -n10000 -j20 ... vtc/fellow_b00062.vtc
completed without any failure.

src/fellow_cache.c

@@ -3974,6 +3974,7 @@ fellow_cache_lru_work(struct worker *wrk, struct fellow_cache_lru *lru)
 			VTAILQ_REMOVE(&lru->lru_head, resume, lru_list);
 			resume->fcs_onlru = 0;
 
+			fco = NULL;
 			oc = NULL;
 			continue;
 		}