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: http://svn.asterisk.org/svn/asterisk/trunk@407752 f38db490-d61c-443f-a65b-d21fe96a405b
chan_iax2: Add some more iaxs[] NULL checks to a routine already full of them.
........

Merged revisions 407764 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 407765 from http://svn.asterisk.org/svn/asterisk/branches/11
........

Merged revisions 407766 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@407767 f38db490-d61c-443f-a65b-d21fe96a405b