Tighten up the management of the do_close flag.

Fixes	#890

bin/varnishd/cache_center.c

@@ -488,7 +488,6 @@ cnt_fetch(struct sess *sp)
 	if (i) {
 		sp->handling = VCL_RET_ERROR;
 		sp->err_code = 503;
-		sp->wrk->do_close = 0;
 	} else {
 		/*
 		 * These two headers can be spread over multiple actual headers
@@ -541,6 +540,7 @@ cnt_fetch(struct sess *sp)
 	}
 
 	/* Clean up partial fetch */
+	AZ(sp->vbc);
 
 	if (sp->objcore != NULL) {
 		CHECK_OBJ_NOTNULL(sp->objcore, OBJCORE_MAGIC);
@@ -729,7 +729,6 @@ cnt_fetchbody(struct sess *sp)
 	/* Use unmodified headers*/
 	i = FetchBody(sp);
 
-	sp->wrk->do_close = 0;
 	sp->wrk->h_content_length = NULL;
 
 	http_Setup(sp->wrk->bereq, NULL);

bin/varnishd/cache_dir.c

@@ -62,6 +62,7 @@ VDI_CloseFd(struct sess *sp)
 	sp->vbc->backend = NULL;
 	VBE_ReleaseConn(sp->vbc);
 	sp->vbc = NULL;
+	sp->wrk->do_close = 0;
 }
 
 /* Recycle a connection ----------------------------------------------*/
@@ -74,6 +75,7 @@ VDI_RecycleFd(struct sess *sp)
 	CHECK_OBJ_NOTNULL(sp->vbc, VBC_MAGIC);
 	CHECK_OBJ_NOTNULL(sp->vbc->backend, BACKEND_MAGIC);
 	assert(sp->vbc->fd >= 0);
+	AZ(sp->wrk->do_close);
 
 	bp = sp->vbc->backend;