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.