libgxps: Stack-based buffer overflow in calling glib in gxps_images_guess_content_type of gcontenttype.c

There is a stack-based buffer over-read in calling GLib in the function gxpsimagesguesscontenttype of gxps-images.c in libgxps through 0.3.0 because it does not reject negative return values from a ginputstreamread call. A crafted input will lead to a remote denial of service attack...