XML: release parser context after parsing

XML: release parser context after parsing

Previously, XML parser contexts were retained with their parsed documents.
When parsing failed in the njs engine, no cleanup was registered, leaking
the context and allowing repeated malformed XML input to grow memory usage
in long-lived NGINX worker processes.

The fix is to limit parser contexts to the parse operation and release
them immediately after parsing or error reporting. The VM cleanup now owns
only the parsed document.