Fix submit after contest

modules/problem.js

@@ -542,6 +542,7 @@ app.post('/problem/:id/submit', async (req, res) => {
     if (contest_id) {
       let contest = await Contest.fromID(contest_id);
       if (!contest) throw new ErrorMessage('无此比赛。');
+      if (!await contest.isRunning()) throw new ErrorMessage('比赛未开始或已结束。');
       let problems_id = await contest.getProblems();
       if (!problems_id.includes(id)) throw new ErrorMessage('无此题目。');