IndexListSlow: fix index comparison & assignment

[sciecode]

I was studying the code and noticed that the IndexListSlow generation code didn't work as expected. The index being used to compare was never recorded and as such the index list would be generated entirely with -1, which isn't valid.

I'm fairly certain this code isn't used much, if at all. Otherwise others would've complained. Especially because malloc fails are very rare on modern systems. Still, better to have it working if the necessity arises.