net: fix timeout with null handle

This commit handles the case where _onTimeout is called with a
null handle.

Refs: https://github.com/nodejs/node/pull/15791
Fixes: https://github.com/nodejs/node/issues/16484
PR-URL: https://github.com/nodejs/node/pull/16489


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>