fix issues reported by Clang Static Analyzer (third attempt)

Author: karel-m

src/pqc/mlkem.c

@@ -480,7 +480,7 @@ static void s_mlkem_poly_reduce(mlkem_poly *r)
 static void s_mlkem_poly_getnoise(mlkem_poly *r, const unsigned char seed[MLKEM_SYMBYTES],
                          unsigned char nonce, int eta)
 {
-   unsigned char buf[3 * MLKEM_N / 4]; /* max for eta=3 */
+   unsigned char buf[3 * MLKEM_N / 4] = {0}; /* max for eta=3 */
    unsigned long buflen = (unsigned long)eta * MLKEM_N / 4;
    s_mlkem_prf(buf, buflen, seed, nonce);
    s_mlkem_poly_cbd_eta(r, buf, eta);