Relax an assertion for the IMS update candidate object

The streaming fetch of an object can fail between lookup time and when
we actually use it. Relax this assertion.

Fixes: #1647

bin/varnishd/cache/cache_req_fsm.c

@@ -426,7 +426,7 @@ cnt_lookup(struct worker *wrk, struct req *req)
 	switch (wrk->handling) {
 	case VCL_RET_DELIVER:
 		if (boc != NULL) {
-			AZ(oc->flags & (OC_F_FAILED|OC_F_PASS));
+			AZ(oc->flags & OC_F_PASS);
 			AZ(oc->exp_flags & OC_EF_DYING);
 			AZ(boc->busyobj);
 			VBF_Fetch(wrk, req, boc, o, VBF_BACKGROUND);