bug#16999: calc crashes when computation limit is increased

[Mattias Engdegård]

reopen 16999
make it so

While the C-level crash has been fixed, the Calc bug (infinite loop) has not.

Whether to signal a domain error for (-2 choose 0) or return the conventional 
value by extension is a matter of taste; I think the latter would be more 
useful. In fact, I see no reason not to extend the function to all integral 
arguments, but would of course value any half-supported opinion on the matter.

Let me attempt a patch, and please forgive my reopening the bug.