Fixed a bug in protobuf read() for non-bool template instantiation.

From: Ben Mahler <benjamin.mahler@gmail.com>
Review: https://reviews.apache.org/r/9368

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1444271 13f79535-47bb-0310-9956-ffa450edef68