fix query requests after dht destruction

index.js

@@ -199,6 +199,7 @@ class DHT extends EventEmitter {
 
   _request (to, command, target, value, onresponse, onerror) {
     const req = this.io.createRequest(to, null, command, target, value)
+    if (req === null) return null
 
     req.onresponse = onresponse
     req.onerror = onerror

lib/query.js

@@ -311,6 +311,10 @@ module.exports = class Query extends Readable {
     this.inflight++
 
     const req = this.dht._request(to, this.command, this.target, this.value, this._onvisitbound, this._onerrorbound)
+    if (req === null) {
+      this.destroy(new Error('Node was destroyed'))
+      return
+    }
     req.oncycle = this._oncyclebound
   }
 }