Fix hashgrid initialization

[PaulWagener]

In the hashgrid the hg->bins is an array of pointers to hg->items. But due to a faulty initialization, the pointers in the highest bins after the final item point (one) outside the range of the hg->items array. If these bins are referenced a segfault can occur.

This change forces the bins pointers to stay within hg->items array.

[skinkie]

We still need some love in the Hashgrid unittest as well.

[PaulWagener]

Added the offending segfaulting test.