If something which looks like a closing tag is left unfinished, but
isn’t paired to an opening tag in the document, the error handling code
would do a null pointer dereference. Avoid that, at the cost of
introducing a new translatable error message.

Includes a test case, courtesy of pdknsk.
Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://gitlab.gnome.org/GNOME/glib/issues/1461