gdk-pixbuf2: excessive memory use

It was found that gdk-pixbuf GIF image loader gdk_pixbuf__gif_image_load() routine did not properly handle certain return values from their subroutines. A remote attacker could provide a specially-crafted GIF image, which once opened in an application, linked against gdk-pixbuf would lead to gdk-pixbuf to return partially initialized pixbuf structure, possibly having huge width and height, leading to that particular application termination due excessive memory use.