libpng: bump version

Fixes memory leak and double-free vulnerability

package/libpng/libpng.mk

@@ -3,7 +3,7 @@
 # libpng (Portable Network Graphic library)
 #
 #############################################################
-LIBPNG_VERSION:=1.2.32
+LIBPNG_VERSION:=1.2.34
 LIBPNG_SITE = http://$(BR2_SOURCEFORGE_MIRROR).dl.sourceforge.net/sourceforge/libpng
 LIBPNG_SOURCE = libpng-$(LIBPNG_VERSION).tar.bz2
 LIBPNG_LIBTOOL_PATCH = NO