Fix a bug in `OctreePointCloudAdjacency::computeNeighbors()`

The `x`, `y`, and `z` fields of `OctreeKey` are unsigned integers, so in some
cases unsigned integer underflow occurs and the tree is queried for
non-existing leaves with huge keys. Due to the way `findLeaf()` is 
implemented, such queries occasionally succeed and return some random leaves.