The go.mod sum was present, but not the module sum.

Change-Id: I7def303bf4507a88dfa1bb0502116efde4d64b01
Reviewed-on: https://go-review.googlesource.com/c/go/+/203882
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>