Segfault in Image.PNM.decode

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=1336

Reference: http://mc.pp.se/hus.ppm

Reported by @marcus

This PPM picture makes PNM.decode dump core. xv can display it.

Pike v7.2 release 10 running Hilfe v2.0 (Incremental Pike Frontend)
> Image.PNM.decode(Stdio.read_file("/home/marcus/mc.pp.se/hus.ppm"));
Segmentation fault (core dumped)