func_audiohookinheritance: Check If A Channel Was Specified

This patch prevents a crash when using the function audiohookinheritance without
setting the channel.

(closes issue ASTERISK-23104)
Reported by: Joel Vandal
Tested by: Joel Vandal
Patches:
    asterisk-23104_audiohook_inherit_no_channel-11.diff
                                     uploaded by Michael L. Young (license 5026)

Review: https://reviewboard.asterisk.org/r/3272/
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Merged revisions 409623 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 409625 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@409626 f38db490-d61c-443f-a65b-d21fe96a405b