From Gentoo. Fixes compilation with libpng-1.5

--- a/src/Util.cpp
+++ b/src/Util.cpp
@@ -79,7 +79,7 @@
     return false;
   }
 
-  if(setjmp(png_ptr->jmpbuf)) {
+  if(setjmp(png_jmpbuf(png_ptr))) {
     png_destroy_write_struct(&png_ptr,NULL);
     fclose(fp);
     return false;
