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


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@411314 f38db490-d61c-443f-a65b-d21fe96a405b