inspector: fix Coverity defects

One defect remains - Coverity believes that a session object is never
freed while in reality its lifespan is tied to a libuv socket.

PR-URL: https://github.com/nodejs/node/pull/10240


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>