src: fix handle leak in Buffer::New()

Fix handle leaks in Buffer::New() and Buffer::Copy() by creating the
handle scope before looking up the env with Environment::GetCurrent().

Environment::GetCurrent() calls v8::Isolate::GetCurrentContext(), which
creates a handle in the current scope, i.e., the scope created by the
caller of Buffer::New() or Buffer::Copy().

PR-URL: https://github.com/nodejs/node/pull/7711


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>