Fix a bug introduced in patch 2 of
https://go-review.googlesource.com/72630 (sense of a map
lookup test was accidentally flipped).

Change-Id: Icc6096ee50be4605fa7542b9fd855c13b8aff090
Reviewed-on: https://go-review.googlesource.com/72850
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>