test: add error checking in callback

Add assert.ifError() for test-dgram-send-callback-buffer-length.

PR-URL: #11446
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: Trott

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

@@ -11,6 +11,7 @@ const offset = 20;
 const len = buf.length - offset;
 
 const messageSent = common.mustCall(function messageSent(err, bytes) {
+  assert.ifError(err);
   assert.notStrictEqual(bytes, buf.length);
   assert.strictEqual(bytes, buf.length - offset);
   client.close();