net: check and throw on error for getsockname

This commit attempts fix a TODO in net.js:
TODO(bnoordhuis) Check err and throw?

PR-URL: https://github.com/nodejs/node/pull/12871


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>