tls: fix assert in context._external accessor

* Restrict the receiver to instances of the FunctionTemplate.
* Use `args.This()` instead of `args.Holder()`.

Fixes: https://github.com/nodejs/node/issues/3682
PR-URL: https://github.com/nodejs/node/pull/5521


Reviewed-By: Fedor Indutny <fedor@indutny.com>