Fixed a memory leak in stout/result.hpp.

The old object needs to be deleted in the assignment operator
overload function.

From: Jie Yu <yujie.jay@gmail.com>
Review: https://reviews.apache.org/r/9512

git-svn-id: https://svn.apache.org/repos/asf/incubator/mesos/trunk@1467379 13f79535-47bb-0310-9956-ffa450edef68