When formatting the error messages for markup parsing errors, the parser
was unconditionally reading a UTF-8 character from the input buffer —
but the buffer might end with a partial code sequence, resulting in
reading off the end of the buffer by up to three bytes.

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

https://gitlab.gnome.org/GNOME/glib/issues/1462