src: check whether inspector is doing io

Inspector start means that it exists, but doesn't mean it is listening
on a port, that only happens if it is doing I/O (i.e. has an io object).

PR-URL: https://github.com/nodejs/node/pull/13504
Fixes: https://github.com/nodejs/node/issues/13499


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>