buildroot

libpng: security bump to version 1.6.16

Fixes a buffer overflow which may allow an attacker to gain write
access to memory.
CVE requested but not yet assigned.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>