Removes unnecessary nil-check when referencing offset from an
address. Suggested by Keith Randall in golang/go#27180.

Updates golang/go#27180

Change-Id: I326ed7fda7cfa98b7e4354c811900707fee26021
Reviewed-on: https://go-review.googlesource.com/131735Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>