adopt save-and-restore pattern for libxml2 error handlers everywhere

[flavorjones]

See #2168 and #2169 for details, but the short version is that we should be more rigorous about saving-and-restoring error handlers and error handler metadata around libxml2 calls, in case any are being made recursively within Nokogiri.

For example, these lines in Nokogiri::HTML::Document::EncodingReader are calling HTML::SAX::PushParser to parse a chunk from the IO read callback of a regular document parse:

nokogiri/lib/nokogiri/html/document.rb

Lines 275 to 277 in 7be6f04

	handler = SAXHandler.new
	parser = Nokogiri::HTML::SAX::PushParser.new(handler)
	parser << chunk rescue Nokogiri::SyntaxError

To allow users to do similarly complex things, we should always save-and-restore the error callbacks (which are the only global state I can think of that we regularly manipulate).

We're doing this in the HTML::SAX::PushParser class to cover ourselves in the aforementioned case:

nokogiri/ext/nokogiri/html_sax_push_parser.c

Lines 24 to 28 in 7be6f04

	Nokogiri_structured_error_func_save_and_set(&handler_state, NULL, NULL);
	status = htmlParseChunk(ctx, chunk, size, Qtrue == _last_chunk ? 1 : 0);
	Nokogiri_structured_error_func_restore(&handler_state);

nokogiri/ext/nokogiri/xml_syntax_error.h

Lines 6 to 17 in 7be6f04

	typedef struct _libxmlStructuredErrorHandlerState {
	void *user_data;
	xmlStructuredErrorFunc handler;
	} libxmlStructuredErrorHandlerState ;
	void init_xml_syntax_error();
	void Nokogiri_structured_error_func_save(libxmlStructuredErrorHandlerState *handler_state);
	void Nokogiri_structured_error_func_save_and_set(libxmlStructuredErrorHandlerState *handler_state,
	void *user_data,
	xmlStructuredErrorFunc handler);
	void Nokogiri_structured_error_func_restore(libxmlStructuredErrorHandlerState *handler_state);

nokogiri/ext/nokogiri/xml_syntax_error.c

Lines 3 to 24 in 7be6f04

	void
	Nokogiri_structured_error_func_save(libxmlStructuredErrorHandlerState *handler_state)
	{
	/* this method is tightly coupled to the implementation of xmlSetStructuredErrorFunc */
	handler_state->user_data = xmlStructuredErrorContext;
	handler_state->handler = xmlStructuredError;
	}
	void
	Nokogiri_structured_error_func_save_and_set(libxmlStructuredErrorHandlerState *handler_state,
	void *user_data,
	xmlStructuredErrorFunc handler)
	{
	Nokogiri_structured_error_func_save(handler_state);
	xmlSetStructuredErrorFunc(user_data, handler);
	}
	void
	Nokogiri_structured_error_func_restore(libxmlStructuredErrorHandlerState *handler_state)
	{
	xmlSetStructuredErrorFunc(handler_state->user_data, handler_state->handler);
	}

This issue is opened to make sure we remember to do this everywhere.

It's somewhat related to wrapping we need to do around any libxml2 callbacks which re-enter the Ruby interpreter and how we handle those exceptions, all of which are detailed at #1610.