security_events: Fix assertion failure in dev-mode on optional IE parsing

When formatting an optional IE, the value is, of course, optional. As such, it
is entirely appropriate for ast_json_object_get to return NULL. If that occurs,
we now simply skip the IE that was requested, as it was not provided by the
entity that raised the event.

Thanks to George Joseph (gtjoseph) for catching this and reporting it in
#asterisk-dev
........

Merged revisions 407750 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@407752 65c4cc65-6c06-0410-ace0-fbb531ad65f3