set client/server socket atomically

lib/io.js

@@ -133,16 +133,16 @@ module.exports = class IO {
   }
 
   async _bindSockets () {
-    this.serverSocket = typeof this._bind === 'function' ? await this._bind() : await bind.udp(this._bind)
+    const serverSocket = typeof this._bind === 'function' ? await this._bind() : await bind.udp(this._bind)
 
     try {
       // TODO: we should reroll the socket is it's close to our preferred range of ports
       // to avoid it being accidentally opened
       // We'll prop need additional APIs for that
       this.clientSocket = await bind.udp()
+      this.serverSocket = serverSocket
     } catch (err) {
-      await new Promise((resolve) => this.serverSocket.close(resolve))
+      await new Promise((resolve) => serverSocket.close(resolve))
-      this.serverSocket = null
       throw err
     }