Handle the case when orig_eax is garbage, for whatever reason.

Another nasty bug that, quite frustratingly, I can't write a sane test for :/.  Adding to the frustration, I have no idea where the weird orig_eax values are coming from, but even if I did, I don't think there's anything we could do about it.

Anyway, after pretty extensive FF testing, this change looks pretty solid.  Resolves #265.  Resolves #266.