incorrect tree_depth

[themighty1]

currently tree depth is incorrectly calculated for power-of-two leaf count. it is +1 than what it should be.

pub fn tree_depth(leaves_count: usize) -> usize {
    8 * core::mem::size_of::<usize>() - leaves_count.leading_zeros() as usize
}

e.g. for leaves_count == 4 this will return 3, whereas it should be 2.

This bug doesn't seem to open any exploitable vectors, it only causes to allocate memory for a non-existent layer.
It is still a good idea to fix this.