inspector: zero out structure members

Ctor has to be added as memset to 0 is no longer an option, since
the structure now has std::vector member.

Attempt at fixing nodejs/node#8155 (so far I was not able to repro it)

PR-URL: https://github.com/nodejs/node/pull/8536


Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>