Fix dialplan function NULL channel safety issues

(closes issue ASTERISK-23391)
Reported by: Corey Farrell
Review: https://reviewboard.asterisk.org/r/3386/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@411313 f38db490-d61c-443f-a65b-d21fe96a405b