This is a minimal fix to prevent this and
other possible future infinite recursion.
We can put in a proper fix for UNC in Go 1.8.

Updates #15879

Change-Id: I3653cf5891bab8511adf66fa3c1a1d8912d1a293
Reviewed-on: https://go-review.googlesource.com/23572
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>