core: discussion: add validation to vnode

packages/hydrooj/src/model/discussion.ts

@@ -280,6 +280,7 @@ export async function getVnode(domainId: string, type: number, id: string, uid?:
     }
     if ([document.TYPE_CONTEST, document.TYPE_TRAINING].includes(type as any)) {
         const model = type === document.TYPE_TRAINING ? training : contest;
+        if (!ObjectId.isValid(id)) throw new DiscussionNodeNotFoundError(id);
         const _id = new ObjectId(id);
         const tdoc = await model.get(domainId, _id);
         if (!tdoc) throw new DiscussionNodeNotFoundError(id);