Avoid printing a second error message when a field of an undefined
variable is accessed.

Fixes #8440.

Change-Id: I3fe0b11fa3423cec3871cb01b5951efa8ea7451a
Reviewed-on: https://go-review.googlesource.com/36751Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>