dns: fix getServers return undefined

PR-URL: nodejs#43922
Reviewed-By: Paolo Insogna <paolo@cowtech.it>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

lib/internal/dns/utils.js

@@ -61,7 +61,7 @@ class Resolver {
   }
 
   getServers() {
-    return ArrayPrototypeMap(this._handle.getServers(), (val) => {
+    return ArrayPrototypeMap(this._handle.getServers() || [], (val) => {
       if (!val[1] || val[1] === IANA_DNS_PORT)
         return val[0];
 
@@ -76,7 +76,7 @@ class Resolver {
     // Cache the original servers because in the event of an error while
     // setting the servers, c-ares won't have any servers available for
     // resolution.
-    const orig = this._handle.getServers();
+    const orig = this._handle.getServers() || [];
     const newSet = [];
 
     ArrayPrototypeForEach(servers, (serv, index) => {

test/parallel/test-dns-get-server.js

@@ -0,0 +1,11 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+
+const { Resolver } = require('dns');
+
+const resolver = new Resolver();
+assert(resolver.getServers().length > 0);
+// return undefined
+resolver._handle.getServers = common.mustCall(() => {});
+assert.strictEqual(resolver.getServers().length, 0);