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: #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>

lib/util.js

@@ -231,10 +231,10 @@ function ensureDebugIsInitialized() {
 
 
 function inspectPromise(p) {
-  ensureDebugIsInitialized();
   // Only create a mirror if the object is a Promise.
   if (!binding.isPromise(p))
     return null;
+  ensureDebugIsInitialized();
   const mirror = Debug.MakeMirror(p, true);
   return {status: mirror.status(), value: mirror.promiseValue().value_};
 }