Nokogiri: Possible Use-After-Free when `Nokogiri::XML::Document#encoding=` raises an exception

Summary

### Summary Calling `Document#encoding=` with an invalid encoding (e.g., a non-string, or a string containing a null byte) raises an exception, but only after freeing the document's current encoding string without replacing it. The document is left referencing freed memory, so the next call to `Document#encoding` reads invalid memory, which can cause a segfault or leak freed bytes into a Ruby `String`. Affects the CRuby (libxml2) implementation only; JRuby is not affected. ### Severity The Nokogiri maintainers have evaluated this as low severity. Reaching it requires an unusual API-usage pattern that does not arise during normal use. The application must pass an invalid encoding to `Document#encoding=`, rescue the resulting exception, and then continue using the same document. Nokogiri 1.19.4 makes this pattern safe with no change to the public API. The document no longer references freed memory after the exception is raised. ### Mitigation Upgrade to Nokogiri 1.19.4 or later. If users are unable to upgrade, avoid passing attacker-controlled values to `Document#encoding=`. Applications that only assign developer-authored encodings are not directly exposed. ### Credit This issue was responsibly reported by Zheng Yu from depthfirst.com.