graphics/gd: Update from 2.2.4 to 2.2.5

This update fixes 2 security issues:

• Double-free in gdImagePngPtr(). (CVE-2017-6362)
• Buffer over-read into uninitialized memory. (CVE-2017-7890)

Full Changelog:
https://github.com/libgd/libgd/blob/gd-2.2.5/CHANGELOG.md

PR: 229707
Submitted by: Mikhail Teterin <mi@FreeBSD.org>
Approved by: maintainer timeout (dinoex, 2 weeks)
MFH: 2018Q3
Security: CVE-2017-6362
Security: CVE-2017-7890