message: Fix assertion in CKey::SignCompact
diff: @@ -246,7 +246,7 @@ bool CKey::SignCompact(const uint256 &hash, std::vector<unsigned char>& vchSig)
  secp256k1_ecdsa_recoverable_signature sig;
  int ret = secp256k1_ecdsa_sign_recoverable(secp256k1_context_sign, &sig, hash.begin(), begin(), secp256k1_nonce_function_rfc6979, nullptr);
  assert(ret);
  - secp256k1_ecdsa_recoverable_signature_serialize_compact(secp256k1_context_sign, &vchSig[1], &rec, &sig);
  + ret = secp256k1_ecdsa_recoverable_signature_serialize_compact(secp256k1_context_sign, &vchSig[1], &rec, &sig);
  assert(ret);
  assert(rec != -1);
  vchSig[0] = 27 + rec + (fCompressed ? 4 : 0);
  