Remove obsolete macros from libpng
https://bugs.webkit.org/show_bug.cgi?id=114817

Patch by Alberto Garcia <agarcia@igalia.com> on 2013-04-18
Reviewed by Benjamin Poulain.

libpng 1.4 removes the png_voidp_NULL and png_error_ptr_NULL
macros. Null pointers must be used directly instead.

* platform/image-encoders/PNGImageEncoder.cpp:
(WebCore::compressRGBABigEndianToPNG):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@148701 268f45cc-cd09-0410-ab3c-d52691b4dbfc