Fix trivial collisions when hashing structures

[someone12469]

To reproduce:

use std::hash::BuildHasher;
use foldhash::fast::RandomState;
//use ahash::RandomState;
//use std::hash::RandomState;
#[derive(Hash)]
struct Point3 {
    x: u64,
    y: u64,
    z: u64
}
#[derive(Hash)]
struct Point4 {
    x: u64,
    y: u64,
    z: u64,
    w: u64
}
fn main() {
    let st = RandomState::default();
    println!("000: {:?}", st.hash_one(Point3 { x: 0, y: 0, z: 0}));
    println!("001: {:?}", st.hash_one(Point3 { x: 0, y: 0, z: 1}));
    println!("002: {:?}", st.hash_one(Point3 { x: 0, y: 0, z: 2}));
    println!("0000: {:?}", st.hash_one(Point4 { x: 0, y: 0, z: 0, w: 0}));
    println!("0010: {:?}", st.hash_one(Point4 { x: 0, y: 0, z: 1, w: 0}));
    println!("0001: {:?}", st.hash_one(Point4 { x: 0, y: 0, z: 0, w: 1}));
    println!("0011: {:?}", st.hash_one(Point4 { x: 0, y: 0, z: 1, w: 1}));
}

Example output (before fix):

000: 8683679690595636920
001: 8683679690595636920
002: 8683679690595636920
0000: 5859751005930861946
0010: 16699619282587451823
0001: 16699619282587451823
0011: 16699619282587451823

This is caused by write_num resetting sponge_len to 0 when it overflows, causing the next finish or write_num to mistakenly assume it's empty. The bug leads to trivial HashDoS for some structures.

[orlp]

That is a very silly bug of me, yes. Thanks for the PR.