Thread.Fifo()->read_array() does not work

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=916

Reported by @marcus

Pike v7.1 release 17 running Hilfe v2.0 (Incremental Pike Frontend)
> object f = Thread.Fifo();
Result: Thread.Fifo(0 / 128)
> f->write(17);
Result: 0
> f->write(42);
Result: 0
> f->read_array();
Bad argument 1 to sizeof().
/i/pike/7.1.17/lib/modules/Thread.pmod:135:
    Thread.Fifo(2 / 128)->read_array()
-:1: ___Foo4711()
>