Set a "noop" illegal callback

The default "illegal" callback calls abort, which will crash the JVM or native app. We check arguments before calling secp256k1 so it should
never happen, except when trying to create a partial musig2 signature with an secret nonce that does not match the private key.

Methods that could be used to verify that the secret nonce does match the private key are not exported, hence the choice to set a custom callback.

jni/c/src/fr_acinq_secp256k1_Secp256k1CFunctions.c

@@ -44,6 +44,11 @@ void JNI_ThrowByName(JNIEnv *penv, const char *name, const char *msg)
     }                                                                          \
   }
 
+static void secp256k1_noop_illegal_callback_fn(const char* str, void* data) {
+   (void)str;
+   (void)data;
+}
+
 /*
  * Class:     fr_acinq_bitcoin_Secp256k1Bindings
  * Method:    secp256k1_context_create
@@ -51,7 +56,9 @@ void JNI_ThrowByName(JNIEnv *penv, const char *name, const char *msg)
  */
 JNIEXPORT jlong JNICALL Java_fr_acinq_secp256k1_Secp256k1CFunctions_secp256k1_1context_1create(JNIEnv *penv, jclass clazz, jint flags)
 {
-  return (jlong)secp256k1_context_create(flags);
+  jlong ctx = (jlong)secp256k1_context_create(flags);
+  secp256k1_context_set_illegal_callback(ctx, &secp256k1_noop_illegal_callback_fn, NULL);
+  return ctx;
 }
 
 /*