Update information for CVE-2017-17858/mupdf

To reviewers: double check this update since the
http://git.ghostscript.com/?p=mupdf.git;a=commit;h=f595e889b91a674eb94db7ca4d832da54f5194cd
is involving and I might have missed something which makes the issue
only be covered before. Before that change though the offsets are
already checked if they are out of range, ofs of type fz_off_t.