http: fix pipeline regression

Always check that socket still has the parser. It may be destroyed
interim, and we may end up with an uncaught exception.

Fix: https://github.com/nodejs/node/issues/3508
PR-URL: https://github.com/nodejs/node-private/pull/5


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>