fix invalid pointer when running with two points

[pnikutta]

When running with two points as input for fit_predict, the bin indices are not computed correctly, leading to

munmap_chunk(): invalid pointer

To reproduce, run the test from this PR or

int main(){
    Dbscan model{0.5, 2};
    auto label_vector = model.fit_predict({{0.f, 0.f}, {1.f, 1.f}});
}

The fix checks and adjusts the bin indices accordingly.