imagemagick triage for jessie

CVE-2019-14981 is an arithmetic exception, security impact is low. Can
still be fixed along with more important patches later, but no-dsa for
now.

CVE-2019-13391 and CVE-2019-13308 would be nice to fix, but the patch
is badly documented and blindly applying a 50+ lines diff won't do any
good.  Wait for upstream to answer questions about the changes.