core: fix problem edit without pid change

packages/hydrooj/package.json

@@ -1,6 +1,6 @@
 {
     "name": "hydrooj",
-    "version": "2.20.5",
+    "version": "2.20.6",
     "bin": "bin/hydrooj.js",
     "main": "dist/loader.js",
     "typings": "dist/loader.d.ts",

packages/hydrooj/src/handler/problem.ts

@@ -328,7 +328,7 @@ export class ProblemEditHandler extends ProblemManageHandler {
         try {
             content = JSON.parse(content);
         } catch { /* Ignore */ }
-        if (await problem.get(domainId, newPid)) throw new BadRequestError('new pid exists');
+        if (newPid !== this.pdoc.pid && await problem.get(domainId, newPid)) throw new BadRequestError('new pid exists');
         const $update: Partial<Pdoc> = {
             title, content, pid: newPid, hidden, tag,
         };