png: Fix some integer overflows
png: Fix some integer overflows

The png loader was not careful enough in some places. Width * height
can overflow an integer.

This should fix http://bugzilla.gnome.org/734556.

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