test: add error checking in callback

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

PR-URL: https://github.com/nodejs/node/pull/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>