Fix dialplan function NULL channel safety issues

(closes issue ASTERISK-23391)
Reported by: Corey Farrell
Review: https://reviewboard.asterisk.org/r/3386/
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Merged revisions 411313 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 411314 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 411315 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@411328 65c4cc65-6c06-0410-ace0-fbb531ad65f3