Fix missing error check in test.

Spotted by Gordon Klass, https://groups.google.com/forum/#!topic/golang-nuts/MdDLbvOjb4o

Change-Id: I22f1a438cbb60a2fe1740fc2d43fbf8aa008b6de
Reviewed-on: https://go-review.googlesource.com/10605Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>