Upgrade libxo to 0.1.6.

Summary of changes:
1.  Coverity defect fixes

Obtained from:  https://github.com/Juniper/libxo/releases/tag/0.1.6


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274672 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f