dgram: pass null as error on successful send()

Prior to c9fd9e21622abb7b3893af457f6aaafb2363ab46, UDP sockets
would callback with a null error on successful send() calls. The
current behavior is to pass 0 as the error. This commit restores
the previous, more expected behavior.

PR-URL: https://github.com/nodejs/node/pull/5929
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>

lib/dgram.js

@@ -392,7 +392,10 @@ function doSend(ex, self, ip, buffer, address, port, callback) {
 function afterSend(err, sent) {
   if (err) {
     err = exceptionWithHostPort(err, 'send', this.address, this.port);
+  } else {
+    err = null;
   }
+
   this.callback(err, sent);
 }
 

test/parallel/test-dgram-send-callback-buffer.js

@@ -9,6 +9,7 @@ const client = dgram.createSocket('udp4');
 const buf = Buffer.allocUnsafe(256);
 
 const onMessage = common.mustCall(function(err, bytes) {
+  assert.strictEqual(err, null);
   assert.equal(bytes, buf.length);
   clearTimeout(timer);
   client.close();