Add error handling for last-resort VDP_END

We lacked error handling in the call to ensure that VDP_END is being sent.

Now we return the first error seen, which can be the one from the last
resort VDP_END call.

Flexelint c2d45aef

Ref #3298

bin/varnishd/storage/storage_simple.c

@@ -243,7 +243,7 @@ sml_iterator(struct worker *wrk, struct objcore *oc,
 	const struct stevedore *stv;
 	ssize_t checkpoint_len = 0;
 	ssize_t len = 0;
-	int ret = 0;
+	int ret = 0, ret2;
 	ssize_t ol;
 	ssize_t nl;
 	ssize_t sl;
@@ -343,8 +343,11 @@ sml_iterator(struct worker *wrk, struct objcore *oc,
 			break;
 	}
 	HSH_DerefBoc(wrk, oc);
-	if ((u & OBJ_ITER_END) == 0)
-		func(priv, OBJ_ITER_END, NULL, 0);
+	if ((u & OBJ_ITER_END) == 0) {
+		ret2 = func(priv, OBJ_ITER_END, NULL, 0);
+		if (ret == 0)
+			ret = ret2;
+	}
 	return (ret);
 }