Only stop the MWI monitor thread if it was actually started.

(closes issue #12086)
Reported by: francesco_r


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@104536 65c4cc65-6c06-0410-ace0-fbb531ad65f3

res/res_smdi.c

@@ -1281,7 +1281,9 @@ static int unload_module(void)
 	ast_cond_signal(&mwi_monitor.cond);
 	ast_mutex_unlock(&mwi_monitor.lock);
 
-	pthread_join(mwi_monitor.thread, NULL);
+	if (mwi_monitor.thread != AST_PTHREADT_NULL) {
+		pthread_join(mwi_monitor.thread, NULL);
+	}
 
 	ast_custom_function_unregister(&smdi_msg_retrieve_function);
 	ast_custom_function_unregister(&smdi_msg_function);