src: fix minor inefficiency in Buffer::New() call

Commit ccb199af ("src: fix deprecation warnings") passes env()->isolate()
to the Buffer::New() call.  It's somewhat inefficient because the callee
looks up the environment from the isolate.  Just pass the env directly.

PR-URL: https://github.com/iojs/io.js/pull/1577


Reviewed-By: Trevor Norris <trev.norris@gmail.com>