libpng-1.2: security update 1.2.54 -> 1.2.55

It should finally fix #11030 and CVE-2015-8126 for 1.2.

pkgs/development/libraries/libpng/12.nix

···
 assert !(stdenv ? cross) -> zlib != null;
 
 stdenv.mkDerivation rec {
-  name = "libpng-1.2.54";
+  name = "libpng-1.2.55";
 
   src = fetchurl {
     url = "mirror://sourceforge/libpng/${name}.tar.xz";
-    sha256 = "0wnjy7gqn0f24qrlggs7kl0ij59by413j1xmqp12n3vqh9j531fg";
+    sha256 = "0zkra0b9lrpk2axassdgkqinmc2ba6b473sm52xbpyknaqs2fljy";
   };
 
   propagatedBuildInputs = [ zlib ];