runtime: use .Pointers() instead of .PtrBytes != 0

Change-Id: I512808d3cf91466eb6f2c21b49b39ff2e08a9b8c
Reviewed-on: https://go-review.googlesource.com/c/go/+/605498


Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>