fix(NODE-5225): eagerly clear MongoClient.topology in MongoClient.close()

At the moment, calling `MongoClient.close()` in quick succession leads
to an error:

```
TypeError: Cannot read properties of undefined (reading 'close')
  at node-mongodb-native/src/mongo_client.ts:530:16
  at new Promise (<anonymous>)
  at LegacyMongoClient.close (src/mongo_client.ts:529:11)
  at processTicksAndRejections (node:internal/process/task_queues:95:5)
  at async Promise.all (index 1)
  at async Context.<anonymous> (test/integration/node-specific/mongo_client.test.ts:393:5)
```

This happens because:

1. The first call attempts to asynchronously [close][1] the session pool
   sessions
2. While this is happening, the second call passes the topology
   [null check][2] (since `this.topology` hasn't yet been [unset][3],
   and tries to also close session pool sessions
3. Both calls then set [`const topology = this.topology`][4] but the
   second call will get undefined since the first call [unset][3]
   `this.topology`
4. The second call now tries to call [`close()`][5] on an `undefined`
   `topology` and throws

This change fixes the issue by moving the `topology` unset immediately
after the `null` check, so we know it's always correctly set.

[1]: https://github.com/mongodb/node-mongodb-native/blob/325c4bc37decdf12e957bfad8bd4ee4d28b1bf95/src/mongo_client.ts#L516
[2]: https://github.com/mongodb/node-mongodb-native/blob/325c4bc37decdf12e957bfad8bd4ee4d28b1bf95/src/mongo_client.ts#L503
[3]: https://github.com/mongodb/node-mongodb-native/blob/325c4bc37decdf12e957bfad8bd4ee4d28b1bf95/src/mongo_client.ts#L527
[4]: https://github.com/mongodb/node-mongodb-native/blob/325c4bc37decdf12e957bfad8bd4ee4d28b1bf95/src/mongo_client.ts#L526
[5]: https://github.com/mongodb/node-mongodb-native/blob/325c4bc37decdf12e957bfad8bd4ee4d28b1bf95/src/mongo_client.ts#L530

Author: alecgibson

src/mongo_client.ts

@@ -504,10 +504,14 @@ export class MongoClient extends TypedEventEmitter<MongoClientEvents> {
       return;
     }
 
+    // clear out references to old topology
+    const topology = this.topology;
+    this.topology = undefined;
+
     // If we would attempt to select a server and get nothing back we short circuit
     // to avoid the server selection timeout.
     const selector = readPreferenceServerSelector(ReadPreference.primaryPreferred);
-    const topologyDescription = this.topology.description;
+    const topologyDescription = topology.description;
     const serverDescriptions = Array.from(topologyDescription.servers.values());
     const servers = selector(topologyDescription, serverDescriptions);
     if (servers.length !== 0) {
@@ -522,10 +526,6 @@ export class MongoClient extends TypedEventEmitter<MongoClientEvents> {
       }
     }
 
-    // clear out references to old topology
-    const topology = this.topology;
-    this.topology = undefined;
-
     await new Promise<void>((resolve, reject) => {
       topology.close({ force }, error => {
         if (error) return reject(error);

test/integration/node-specific/mongo_client.test.ts

@@ -385,6 +385,14 @@ describe('class MongoClient', function () {
     }
   });
 
+  it('it can call close() concurrently', async function () {
+    const client = this.configuration.newClient();
+    await client.connect();
+    // Ensure topology is opened before trying to close
+    await client.db().command({ hello: 1 });
+    await Promise.all([client.close(), client.close()]);
+  });
+
   context('explict #connect()', () => {
     let client: MongoClient;
     beforeEach(function () {