src: fix memory leak in ExternString

v8 will silently return an empty handle
which doesn't delete our data if string length is
above String::kMaxLength

Fixes: https://github.com/nodejs/node/issues/1374
PR-URL: https://github.com/nodejs/node/pull/2402


Reviewed-By: trevnorris - Trevor Norris <trev.norris@gmail.com>
Reviewed-By: indutny - Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>

Amended by @rvagg to change author date from
  "1970-08-16 16:09:02 +0200"
to
  "2015-08-16 16:09:02 +0200"
as per discussion @ https://github.com/nodejs/node/issues/2713