size of buffer in kyber768/m4fstack/matacc.c (and probably m4fspeed too)

[marco-palumbi]

unsigned char buf[XOF_BLOCKBYTES+2]; could be of size XOF_BLOCKBYTES (see: https://github.com/mupq/pqm4/blob/master/crypto_kem/kyber768/m4fstack/matacc.c#L20)

if I'm not wrong the code relay on XOF_BLOCKBYTES being multiple of 3 (see pq-crystals/kyber@ce492cd) and xof_squeezeblocks() is always called with the address of the first byte of buf (see also https://github.com/mupq/pqm4/blob/master/crypto_kem/kyber768/m4fstack/matacc.i#L101)

buf can then be defined: unsigned char buf[XOF_BLOCKBYTES];
the same should apply to the m4fspeed version.
Can please check?