pnm: Fix crash opening images with overly large dimensions
pnm: Fix crash opening images with overly large dimensions

Internally, the PNM loader stores the width and height as unsigned
integers, but the external callback to the application,
GdkPixbufModuleSizeFunc, uses integers. So the maximum width and height
really are MAXINT, not MAXUINT.

https://bugzilla.gnome.org/show_bug.cgi?id=775232

GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf