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
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Merged revisions 409626 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@409627 65c4cc65-6c06-0410-ace0-fbb531ad65f3

funcs/func_audiohookinherit.c

@@ -228,6 +228,11 @@ static int func_inheritance_write(struct ast_channel *chan, const char *function
 		return -1;
 	}
 
+	if (!chan) {
+		ast_log(LOG_WARNING, "No channel was provided to INHERITANCE function.\n");
+		return -1;
+	}
+
 	allow = ast_true(value);
 
 	/* Step 2: retrieve or set up datastore */