Print "<unknown>" for artificial endpoint in PJSIP security events.

Previously, this printed a UUID, which was not very clear when dealing
with an artificial endpoint.

Review: https://reviewboard.asterisk.org/r/3113
........

Merged revisions 405298 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@405299 f38db490-d61c-443f-a65b-d21fe96a405b