Hasher expects correct output length

[zx2c4]

Symmetric state has h of a fixed maximum size:

    SymmetricState{
        cipherstate: cipherstate,
        hasher: hasher,
        h: [0u8; MAXHASHLEN],
        ck : [0u8; MAXHASHLEN],
        has_key: false,
        has_preshared_key: false,
    }

When initializing, the following code is run:

fn initialize(&mut self, handshake_name: &[u8]) {
    if handshake_name.len() <= self.hasher.hash_len() {
        self.h = [0u8; MAXHASHLEN];
        copy_memory(handshake_name, &mut self.h);
    } else {
        self.hasher.reset();
        self.hasher.input(handshake_name);
        self.hasher.result(&mut self.h);
    }

When using PSK mode and a 32-byte hash function, such as Blake2s, the second branch is taken:

        self.hasher.reset();
        self.hasher.input(handshake_name);
        self.hasher.result(&mut self.h);

This passes self.h to hasher.result, which is 64 bytes:

        h: [0u8; MAXHASHLEN],

Unfortunately, the result function expects for h to be equal to the output length. That is, it expects h to be 32 bytes. From blake2s.rs:

fn finalize( &mut self, out: &mut [u8] ) {
    assert!(out.len() == self.digest_length as usize);

This makes using Blake2s with PSK impossible. The solution is to have the size of h change based on which hash function is being used.