Crash when parsing malformed XML after rxkb_context_parse() due to error handler not being reset

[sbstnk]

We got a report of gnome-shell crashing when libgweather is trying to parse malformed XML after gnome-shell has previously called (via gnome-desktop) something like:

rxkb_context_new();
rxkb_context_parse();
rxkb_context_unref();

rxkb_context_parse() calls xmlSetGenericErrorFunc() via parse() to set the global libxml2 error handler to its xml_error_func(), which depends on a valid rxkb_context. The context however is no longer valid after rxkb_context_unref(), while the error handler is still pointing to xml_error_func() . This then results in a crash whenever something causes libxml2 to trigger an error afterwards, such as libgweather in the gnome-shell report.

parse() should restore the default error handler after it is done parsing.

[wismill]

Thanks for the great debugging and the corresponding PR!