Fix circular structure returned for unclosed tags - #224

Fixes #223.

An unclosed tag left the parser's internal parent back-reference attached to the returned object, so the result was circular:

convert.xml2json('<a>');
// TypeError: Converting circular structure to JSON

convert.xml2js('<a>').elements[0].parent === result;   // true

That also contradicts the README, which documents parent as appearing only "by setting {addParent: true}".

Cause

lib/xml2js.js:250 attaches the reference with the comment "will be deleted in onEndElement() if !options.addParent". onEndElement is the only place that removes it, and it never runs for a tag that is never closed.

The sax error that should have surfaced can't: "Unclosed root tag" is raised inside end(), after write()'s entry-time error check, and onError (line 312) is error.note = error, which discards it. Hence the asymmetry — <a></b> throws a clean Unmatched closing tag: b because that fires during write(), while <a> does not.

The change

Unwinds anything left open after parsing, mirroring the cleanup already in onEndElement.

Deliberately conservative: it preserves the library's lenient "don't throw on unclosed tags" behaviour rather than touching onError, so nothing that parses today starts throwing. It terminates safely if the chain is broken, and addParent: true still attaches parent exactly as documented.

Tests

Existing suite unchanged: 2363 specs, 0 failures. Three specs added, covering the leak, serialisability, and that addParent: true still works — 2366 total.

Confirmed they catch it. Reverting only lib/xml2js.js:

Failures:
1) …unclosed tags leaving a circular parent reference should not leak the internal parent key when addParent is false
   Expected true to be false.

Note

Truncated XML is ordinary production input — a cut-off feed, a partial upload, a socket closed mid-response. The current failure surfaces as a JSON.stringify error far from the parse, or as an object that silently breaks deep equality, structured cloning and res.json().