CVE-2018-5360/tiff added

Note that the issue is claimed to be fixed in a later/latest version of
libtiff. But the reporter does not provide any more information than:

> Thanks for your reply.
> We have checked the issue carefully and found it is a bug in libtiff,
> which got fixed in the latest version of libtiff.
> Thanks again.

Cf. https://sourceforge.net/p/graphicsmagick/bugs/540/#19db/0e4d/5217/a555