xml.sax expatreader: entity stack not cleaned up after error in external_entity_ref()

Follow-up from gh-148427 / PR #148435.

In ExpatParser.external_entity_ref() (Lib/xml/sax/expatreader.py), when an exception occurs during external entity parsing, the _entity_stack cleanup (restoring self._parser and self._source) is skipped because except Exception: return 0 exits the method before reaching lines 430–431:

try:
    xmlreader.IncrementalParser.parse(self, source)
except Exception:
    return 0  # exits here, cleanup below never runs

(self._parser, self._source) = self._entity_stack[-1]  # skipped
del self._entity_stack[-1]                               # skipped
return 1

After an error, self._parser still points to the sub-parser for the external entity, self._source points to the entity's input source, and the original parser/source pair is stuck in _entity_stack.

In practice this probably doesn't matter much since the parser is usually abandoned after a SAXParseException, but it's still a state corruption if someone tries to reuse the parser or inspect its state after catching the exception.

A finally clause would fix this, but that was intentionally kept out of gh-148427 to keep the scope minimal.

Start by reading ExpatParser.external_entity_ref() in Lib/xml/sax/expatreader.py and the related gh-148427 / PR #148435 context. Done means an exception during external entity parsing no longer leaves self._parser, self._source, or _entity_stack in the corrupted state described.