Prior to  c9fd9e2 , 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: nodejs#5929
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>