Fix an undefined behaviour problem

Fix an undefined behaviour problem in engine_split_particles() by using the more expensive (but correct) functions to relink the gpart to other particles.

Fix #646 (closed).

Thanks does seem to squash the error.

Makes sense since the difference between two array pointers doesn't necessarily equate to an exact number of struct gparts in this case, so the offset could be one less than expected I guess.

And of course, padding is exactly the sort of thing that varies between compilers.

Accepting, although I suspect it is slower, but correct.

merged

mentioned in commit 29d124d4

Indeed. I have a parallel version of this coming up soon.

mentioned in commit 17e3d5cf