stream_wrap: do not crash if handle was closed

Ignore cases where the handle is already gone, like we do in
`handle_wrap.cc`. It should be safe to close handle and then call some
binding methods on it, since the internal handle may be shared between
`_tls_wrap.js` and `net.js` modules.

Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
PR-URL: https://github.com/node-forward/node/pull/37