util: don't init Debug if it's not needed yet

Because any call to util.inspect() with an object results in
inspectPromise() being called, Debug was being initialized even when
it's not needed. Instead, the initialization is placed after the
isPromise check.

PR-URL: https://github.com/nodejs/node/pull/8452


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>