Fix stale gzip preallocated pointer

[xintenseapple]

Summary:

• Clear the gzip filter preallocated-memory pointer immediately after freeing it in the normal deflate-end path.

Verification:

• git diff --check; ./auto/configure --without-http_rewrite_module; make -j2; ./objs/nginx -t -p "$PWD" -c conf/nginx.conf

Fixes #25

[mdounin]

The patch itself is trivial and looks fine, though I would like to include some additional details in the commit log. Suggested text can be found here:

https://freenginx.org/pipermail/nginx-devel/2026-June/001188.html