Bugfix: don't create an announcer if closing

[HDegroote]

The race condition triggered when server.listen() created the announcer

hyperdht/lib/server.js

Lines 111 to 116 in 75059f5

	await this.dht.bind()
	this.target = hash(keyPair.publicKey)
	this._keyPair = keyPair
	this._announcer = new Announcer(this.dht, keyPair, this.target, opts)

after server.close() started executing but before await this.dht.bind()resolved, because then this._listening is not yet true, andclose() early returns if no listener is found:

hyperdht/lib/server.js

Lines 83 to 86 in 75059f5

	async _close () {
	this.closed = true
	if (!this._listening) return

It's a catastrophic bug when it triggers, because an endless loop floods the CPU in Announcer._background():

hyperdht/lib/announcer.js

Lines 96 to 97 in 75059f5

	async _background () {
	while (!this.stopped) {

It will continuously throw NODE_DESTROYED errors which are caught and ignored