zlib: fix memory leak for unused zlib instances

An oversight in an earlier commit led to a memory leak
in the untypical situation that zlib instances are created
but never used, because zlib handles no longer started
out their life as weak handles.

The bug was introduced in bd201102.

Refs: https://github.com/nodejs/node/pull/20455

PR-URL: https://github.com/nodejs/node/pull/21607


Reviewed-By: Tobias Nießen <tniessen@tnie.de>