src: fix handle leak in UDPWrap::Instantiate()

Create a handle scope before performing a check that creates a handle,
otherwise the handle is leaked into the handle scope of the caller.

PR-URL: https://github.com/nodejs/node/pull/7711


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>