gdk-pixbuf-io: Check length before checking for pixdata magic
gdk-pixbuf-io: Check length before checking for pixdata magic

https://bugzilla.gnome.org/show_bug.cgi?id=784583

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