call manager_event only if there is not null channel structure

(Closes issue ASTERISK-19298)
Reported by: robinfood
Patches:
        issue19298.patch uploaded by may213 (License #5415)
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Merged revisions 355136 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 355137 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@355138 65c4cc65-6c06-0410-ace0-fbb531ad65f3