message: Fix witness signature always panicking with out-of-index
diff: @@ -65,7 +65,7 @@ impl WitnessAccountData {
  spending_counter: &account::SpendingCounter,
  ) -> Self {
  let mut v = Vec::with_capacity(65);
  - v[0] = WITNESS_TAG_ACCOUNT;
  + v.push(WITNESS_TAG_ACCOUNT);
  v.extend_from_slice(block0.as_ref());
  v.extend_from_slice(transaction_id.as_ref());
  v.extend_from_slice(&spending_counter.to_bytes());
  