Fix a crash in chan_sip. Don't try to stop the monitor thread if it was never

started.  (closes issue #10124, reported by gzero, fixed by me)


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@73598 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_sip.c

@@ -17642,7 +17642,7 @@ static int unload_module(void)
 	ast_mutex_unlock(&iflock);
 
 	ast_mutex_lock(&monlock);
-	if (monitor_thread && (monitor_thread != AST_PTHREADT_STOP)) {
+	if (monitor_thread && (monitor_thread != AST_PTHREADT_STOP) && (monitor_thread != AST_PTHREADT_NULL)) {
 		pthread_cancel(monitor_thread);
 		pthread_kill(monitor_thread, SIGURG);
 		pthread_join(monitor_thread, NULL);