Fix a deadlock that occurs when func_volume is used on a local channel.

This was discovered by trying to perform a call forward to an extension
that makes use of func_volume. When the local channel is optimized away,
the datastore on the local;2 channel would have its audiohook destroyed
rather than detaching the audiohook from the channel and then destroying
it.

With this patch, func_volume's datastore destructor takes the proper
route of detaching the audiohook and then destroying it.

(closes issue ASTERISK-19611)
reported by Volker Sauer
Patches:
	ASTERISK-19611.patch uploaded by Mark Michelson (license #5049)
........

Merged revisions 368898 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@368899 65c4cc65-6c06-0410-ace0-fbb531ad65f3

funcs/func_volume.c

@@ -90,6 +90,9 @@ static void destroy_callback(void *data)
 	struct volume_information *vi = data;
 
 	/* Destroy the audiohook, and destroy ourselves */
+	ast_audiohook_lock(&vi->audiohook);
+	ast_audiohook_detach(&vi->audiohook);
+	ast_audiohook_unlock(&vi->audiohook);
 	ast_audiohook_destroy(&vi->audiohook);
 	ast_free(vi);