dns/dnsmessage: tweak the error value on nil resource body

Change-Id: I17444d1cc3ce03b6947aaae18d4cecdf9c36193f
Reviewed-on: https://go-review.googlesource.com/47110
Reviewed-by: Ian Gudger <igudger@google.com>
Reviewed-by: Matt Layher <mdlayher@gmail.com>
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
2 files changed