src: remove unprofessional slang in assertions

Convert `CHECK(0 && "wtf?")` (sic) assertions to more suitable
`UNREACHABLE()` macro invocations in `node_zlib.cc`.

PR-URL: https://github.com/nodejs/node/pull/17166


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>