dgram: do not emit error for failed DNS lookups in send()

Modifies the dgram send() method to not emit errors when a DNS lookup
fails if there is no callback. Given that the same UDP socket can be
used to send messages to different hosts, the socket can be reused even
if one of those send() fails.

This slightly changes the behavior of a stable API, so that it behaves
as users would expect to.

This is based of @chrisdickinson nodejs/node-v0.x-archive#7738.
Discussion in nodejs/node-v0.x-archive#4846.

lib/dgram.js

@@ -299,12 +299,8 @@ Socket.prototype.send = function(buffer,
 
   self._handle.lookup(address, function(ex, ip) {
     if (ex) {
-      if (callback) {
+      if (typeof callback === 'function') {
         callback(ex);
-
-        if (self.listeners('error').length)
-          self.emit('error', ex);
-
         return;
       }
 

test/internet/test-dgram-send-cb-quelches-error.js

@@ -0,0 +1,37 @@
+'use strict';
+var common = require('../common');
+var mustCall = common.mustCall;
+var assert = require('assert');
+var dgram = require('dgram');
+var dns = require('dns');
+
+var socket = dgram.createSocket('udp4');
+var buffer = new Buffer('gary busey');
+
+dns.setServers([]);
+
+socket.once('error', onEvent);
+
+// assert that:
+// * callbacks act as "error" listeners if given.
+// * error is never emitter for missing dns entries
+//   if a callback that handles error is present
+// * error is emitted if a callback with no argument is passed
+socket.send(buffer, 0, buffer.length, 100,
+            'dne.example.com', mustCall(callbackOnly));
+
+function callbackOnly(err) {
+  assert.ok(err);
+  socket.removeListener('error', onEvent);
+  socket.on('error', mustCall(onError));
+  socket.send(buffer, 0, buffer.length, 100, 'dne.example.com');
+}
+
+function onEvent(err) {
+  assert.fail('Error should not be emitted if there is callback');
+}
+
+function onError(err) {
+  assert.ok(err);
+  socket.close();
+}