fix race condition involving engine.destroy() and ready event

The 'ready' event sets up an interval timer that is cleared in
`destroy()`. However, in some situations, it is possible to for the
interval to be created after `destroy()` has run, resulting in an
interval timer that is never cleared, and thus an app that does not
exit when it is supposed to. This change fixes the issue and adds a
test for it.

Refs: mafintosh#198

index.js

@@ -554,7 +554,9 @@ var torrentStream = function (link, opts, cb) {
         onupdate()
       }
 
-      rechokeIntervalId = setInterval(onrechoke, RECHOKE_INTERVAL)
+      if (!destroyed) {
+        rechokeIntervalId = setInterval(onrechoke, RECHOKE_INTERVAL)
+      }
 
       process.nextTick(function () {
         engine.emit('torrent', torrent)

test/tracker.js

@@ -108,6 +108,32 @@ test('peer should connect to an alternate tracker', function (t) {
   server.listen(54321)
 })
 
+// This test seems like it might be better contained in basic.js but I am unable
+// to trigger the bug without the addition of the server, so it is much more
+// like tests in tracker.js, so that's where I'm putting it. ¯\_(ツ)_/¯
+test('calling destroy() before \'ready\' should not hold event loop open', function (t) {
+  t.plan(5)
+  var engine = null
+  var server = new tracker.Server()
+  server.once('listening', function () {
+    t.ok(true, 'tracker should be listening')
+
+    engine = torrents(torrent, { dht: false, trackers: ['http://127.0.0.1:54321/announce'] })
+    engine.destroy(function () {
+      engine.remove(t.ok.bind(t, true, 'should be destroyed'))
+    })
+    engine.once('ready', function () {
+      t.ok(true, 'should be ready')
+    })
+  })
+  server.once('start', function (addr) {
+    t.equal(addr, '127.0.0.1:6881')
+
+    server.close(t.ok.bind(t, true, 'tracker should be closed'))
+  })
+  server.listen(54321)
+})
+
 test('cleanup', function (t) {
   t.plan(2)
   fixture.destroy(t.ok.bind(t, true, 'should be destroyed'))