fuzz: BIP324: damage ciphertext/aad in full byte range

Currently the damaging of input data for decryption (either ciphertext
or aad) only ever happens in the lower nibble within the byte at the
damage position, as the bit position for the `damage_val` byte was
calculated with `damage_bit & 3` (corresponding to `% 4`) rather than
`damage_bit & 7` (corresponding to the expected `% 8`).

Author: theStack

src/test/fuzz/bip324.cpp

@@ -98,7 +98,7 @@ FUZZ_TARGET(bip324_cipher_roundtrip, .init=Initialize)
             unsigned damage_bit = provider.ConsumeIntegralInRange<unsigned>(0,
                 (ciphertext.size() + aad.size()) * 8U - 1U);
             unsigned damage_pos = damage_bit >> 3;
-            std::byte damage_val{(uint8_t)(1U << (damage_bit & 3))};
+            std::byte damage_val{(uint8_t)(1U << (damage_bit & 7))};
             if (damage_pos >= ciphertext.size()) {
                 aad[damage_pos - ciphertext.size()] ^= damage_val;
             } else {