Overflow in the allocation of extra sparts

@matthieu I am not sure how to deal with it (and if we should).

Here if you have a very large number of extra sparts (easily done as we do number_cells * expected_stars_per_cell), you can overflow and fail on the next bzero (segmentation fault). https://gitlab.cosma.dur.ac.uk/swift/swiftsim/blob/master/src/space.c#L966

How do you want to deal with it?