src: fix invalid windowBits=8 gzip segfault

`{ windowBits: 8 }` is legal for deflate streams but not gzip streams.
Fix a nullptr dereference when formatting the error message.

Bug introduced in commit c34eae5f ("zlib: refactor zlib internals")
from September 2018.

PR-URL: https://github.com/nodejs/node/pull/33045


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: David Carlier <devnexen@gmail.com>