Add a safeguard to prevent infinite loop

[viroulep]

Once I messed up group assignments and ended up in an infinite loop because the group assignment solver would not find a solution.
Have you encountered this issue before?

Here I'm not sure about the proper fix: I definitely want compscript to complain if groups are not feasible, but at the same time an infinite loop does not sound good.
For now I've simply added a break with a log, which may not be bothering enough (or let's say one could miss it), do you have a suggestion on how to return a proper error from this part of the code?

[timreyn]

Yes, this has happened many times to me... :/

This is an improvement for now. When I start working on NAC groups I'll try to make this more resilient.