Update status for CVE-2017-18220, needs clarification

Mar as fixed with the version adding 98721124e51f. The issue description
is the CloseBlob use-after-free and
https://sourceforge.net/p/graphicsmagick/bugs/438/
indicates from upstream that the issue is fixed with
http://hg.graphicsmagick.org/hg/GraphicsMagick/rev/98721124e51f whereas
a reporter claims a fifferent fix, which is related to CloseBlob use.

Needs a closer review.