We should only unsubscribe to the device state event

subscription if we have previously subscribed. Otherwise
a segfault will occur.

(closes issue #13476)
Reported by: jonnt
Patches:
      13476.patch uploaded by putnopvut (license 60)
Tested by: jonnt



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@143609 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_agent.c

@@ -2535,7 +2535,9 @@ static int unload_module(void)
 	/* First, take us out of the channel loop */
 	ast_channel_unregister(&agent_tech);
 	/* Delete devicestate subscription */
-	agent_devicestate_sub = ast_event_unsubscribe(agent_devicestate_sub);
+	if (agent_devicestate_sub) {
+		agent_devicestate_sub = ast_event_unsubscribe(agent_devicestate_sub);
+	}
 	/* Unregister dialplan functions */
 	ast_custom_function_unregister(&agent_function);	
 	/* Unregister CLI commands */