inspector: Do not crash if the port is n/a

Node process will no longer terminate with an assertion if the
inspector port is not available.

PR-URL: https://github.com/nodejs/node/pull/7874


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