zlib: fix use after null when calling .close

An internal zlib error may cause _handle to be set to null.
Close now will check if there is a _handle prior to calling .close on
it.

PR-URL: https://github.com/nodejs/node/pull/5982
Fixes: https://github.com/nodejs/node/issues/6034


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>