libpng: update to 1.2.57

Fixes CVE-2016-10087

Signed-off-by: Daniel Albers <Daniel.Albers@public-files.de>

libs/libpng/Makefile

@@ -8,12 +8,12 @@
 include $(TOPDIR)/rules.mk
 
 PKG_NAME:=libpng
-PKG_VERSION:=1.2.56
+PKG_VERSION:=1.2.57
 PKG_RELEASE:=1
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.xz
 PKG_SOURCE_URL:=@SF/libpng
-PKG_MD5SUM:=868562bd1c58b76ed8703f135a2e439a
+PKG_MD5SUM:=307052e5e8af97b82b17b64fb1b3677a
 PKG_MAINTAINER:=Jo-Philipp Wich <jo@mein.io>
 
 PKG_LICENSE:=Libpng GPL-2.0+ BSD-3-Clause