Fix return value of xml_parse(_into_struct) for recursive parsing

As of PHP 8.0.0, these functions are supposed to return int, so we
cannot return `false`.  Since calling the parser recursively is a
programmer error, we throw an `Error` in this case.

Cf. <#7363>.

ext/xml/tests/bug73135.phpt

@@ -21,10 +21,6 @@ HERE;
     xml_parse($parser, $xml);
 ?>
 --EXPECTF--
-Warning: xml_parse(): Parser must not be called recursively in %s%ebug73135.php on line %d
-
-Warning: xml_parse(): Parser must not be called recursively in %s%ebug73135.php on line %d
-
-Warning: xml_parse(): Unable to call handler ahihi() in %s%ebug73135.php on line %d
-
-Warning: xml_parse(): Unable to call handler ahihi() in %s%ebug73135.php on line %d
+Fatal error: Uncaught Error: Parser must not be called recursively in %s:%d
+Stack trace:
+%a

ext/xml/xml.c

@@ -1258,8 +1258,8 @@ PHP_FUNCTION(xml_parse)
 
 	parser = Z_XMLPARSER_P(pind);
 	if (parser->isparsing) {
-		php_error_docref(NULL, E_WARNING, "Parser must not be called recursively");
-		RETURN_FALSE;
+		zend_throw_error(NULL, "Parser must not be called recursively");
+		RETURN_THROWS();
 	}
 	parser->isparsing = 1;
 	ret = XML_Parse(parser->parser, (XML_Char*)data, data_len, isFinal);