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/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@409623 f38db490-d61c-443f-a65b-d21fe96a405b