inspector: check if connected before waiting

Fixes: https://github.com/nodejs/node/issues/10093
PR-URL: https://github.com/nodejs/node/pull/10094


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