Contribs: update libpng to 1.6.16

CVE-2014-9495

(cherry picked from commit ed62e5b38363e6ecbf9c234c37510bf87db5c3c4)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

contrib/src/png/SHA512SUMS

-18cd8f59f5920a04f95e6b200affa6a2a9091a79b3e8d01931c95bad1680495f189c87cb912573c23a6681bf764cd981a97b5efc502cf90892b434db422b1892  libpng-1.6.14.tar.xz
+d469a86b8c111be98c15afb1744ead0c7235dbd0b18cf47c5b4b4cfe64c7b1f63897d32e1fc65b0fdd2e01aa6767cad85ea3c0f6aabf9b647696e58803904fa3  libpng-1.6.16.tar.xz

contrib/src/png/rules.mak

 # PNG
-PNG_VERSION := 1.6.14
+PNG_VERSION := 1.6.16
 PNG_URL := $(SF)/libpng/libpng16/$(PNG_VERSION)/libpng-$(PNG_VERSION).tar.xz
 
 PKGS += png