message: JNI: fix use sig array
  Summary: This is a backport of secp256k1 PR583
  Test Plan: I don't have a good setup to test the java code, so I'll trust CI on that one.
  Reviewers: #bitcoin_abc, Fabien
diff: @@ -81,15 +81,15 @@ SECP256K1_API jobjectArray JNICALL Java_org_bitcoin_NativeSecp256k1_secp256k1_1e
  jbyteArray sigArray, intsByteArray;
  unsigned char intsarray[2];
  - secp256k1_ecdsa_signature sig[72];
  + secp256k1_ecdsa_signature sig;
  - int ret = secp256k1_ecdsa_sign(ctx, sig, data, secKey, NULL, NULL);
  + int ret = secp256k1_ecdsa_sign(ctx, &sig, data, secKey, NULL, NULL);
  unsigned char outputSer[72];
  size_t outputLen = 72;
  if( ret ) {
  - int ret2 = secp256k1_ecdsa_signature_serialize_der(ctx,outputSer, &outputLen, sig ); (void)ret2;
  + int ret2 = secp256k1_ecdsa_signature_serialize_der(ctx,outputSer, &outputLen, &sig ); (void)ret2;
  }
  intsarray[0] = outputLen;
  