fix: fix race condition when leaving rooms (#74)

This should fix the related issue:

```
TypeError: Cannot read property 'size' of undefined
    at Adapter._del (/app/node_modules/socket.io-adapter/dist/index.js:67:37)
    at Adapter.delAll (/app/node_modules/socket.io-adapter/dist/index.js:83:18)
    at Socket.leaveAll (/app/node_modules/socket.io/dist/socket.js:190:22)
    at Socket._onclose (/app/node_modules/socket.io/dist/socket.js:334:14)
    at Client.onclose (/app/node_modules/socket.io/dist/client.js:245:20)
    at Socket.emit (events.js:412:35)
    at Socket.onClose (/app/node_modules/engine.io/lib/socket.js:348:12)
    at Object.onceWrapper (events.js:519:28)
    at WebSocket.emit (events.js:400:28)
    at WebSocket.onClose (/app/node_modules/engine.io/lib/transport.js:106:10)
```

A test case was added, which reproduces the issue by adding a listener
to the "leave-room" event. This does not seem to be the case for the
user reporting the issue though, which may indicate that the root cause
is elsewhere.

lib/index.ts

@@ -82,14 +82,14 @@ export class Adapter extends EventEmitter {
     this._del(room, id);
   }
 
-  private _del(room, id) {
-    if (this.rooms.has(room)) {
-      const deleted = this.rooms.get(room).delete(id);
+  private _del(room: Room, id: SocketId) {
+    const _room = this.rooms.get(room);
+    if (_room != null) {
+      const deleted = _room.delete(id);
       if (deleted) {
         this.emit("leave-room", room, id);
       }
-      if (this.rooms.get(room).size === 0) {
-        this.rooms.delete(room);
+      if (_room.size === 0 && this.rooms.delete(room)) {
         this.emit("delete-room", room);
       }
     }

test/index.js

@@ -230,6 +230,16 @@ describe("socket.io-adapter", () => {
       adapter.del("s1", "r1");
     });
 
+    it("should not throw when calling del twice", done => {
+      const adapter = new Adapter({ server: { encoder: null } });
+      adapter.on("leave-room", (room, sid) => {
+        adapter.del("s1", "r1");
+        process.nextTick(done);
+      });
+      adapter.addAll("s1", new Set(["r1"]));
+      adapter.del("s1", "r1");
+    });
+
     it("should emit a 'leave-room' event with delAll method", done => {
       const adapter = new Adapter({ server: { encoder: null } });
       adapter.on("leave-room", (room, sid) => {