Update information for CVE-2018-4868/exiv2

The vulnerable code was introduced in 0.26, after and with commits
around

https://github.com/Exiv2/exiv2/commit/699e1c744e50782e3ed7411cc6ac28260aa169c0

The proposed fix is to add a check DataBuf.size_ in
Jp2Image::readMetadata().