message: Guard memcmp in tests against mixed size inputs.
  Summary:
  Reported by real-or-random.
  Fixes
  This is a backport of secp256k1 PR627
  Test Plan:
  make check
  Reviewers: #bitcoin_abc, Fabien
diff: @@ -4315,7 +4315,7 @@ int test_ecdsa_der_parse(const unsigned char *sig, size_t siglen, int certainly_
  if (valid_der) {
  ret |= (!roundtrips_der_lax) << 12;
  ret |= (len_der != len_der_lax) << 13;
  - ret |= (memcmp(roundtrip_der_lax, roundtrip_der, len_der) != 0) << 14;
  + ret |= ((len_der != len_der_lax) || (memcmp(roundtrip_der_lax, roundtrip_der, len_der) != 0)) << 14;
  }
  ret |= (roundtrips_der != roundtrips_der_lax) << 15;
  if (parsed_der) {
  @@ -4356,7 +4356,7 @@ int test_ecdsa_der_parse(const unsigned char *sig, size_t siglen, int certainly_
  ret |= (roundtrips_der != roundtrips_openssl) << 7;
  if (roundtrips_openssl) {
  ret |= (len_der != (size_t)len_openssl) << 8;
  - ret |= (memcmp(roundtrip_der, roundtrip_openssl, len_der) != 0) << 9;
  + ret |= ((len_der != (size_t)len_openssl) || (memcmp(roundtrip_der, roundtrip_openssl, len_der) != 0)) << 9;
  }
  #endif
  return ret;
  