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.

tests/src/commonTest/kotlin/fr/acinq/secp256k1/Secp256k1Test.kt

@@ -520,6 +520,29 @@ class Secp256k1Test {
                 -1
             )
         }
+        assertFails {
+            val privkeys = listOf(
+                "0101010101010101010101010101010101010101010101010101010101010101",
+                "0202020202020202020202020202020202020202020202020202020202020202",
+            ).map { Hex.decode(it) }.toTypedArray()
+            val pubkeys = privkeys.map { Secp256k1.pubkeyCreate(it) }
+
+            val sessionId = Hex.decode("0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F0F")
+            val nonces = pubkeys.map { Secp256k1.musigNonceGen(sessionId, null, it, null, null, null) }
+            val secnonces = nonces.map { it.copyOfRange(0, 132) }
+            val pubnonces = nonces.map { it.copyOfRange(132, 132 + 66) }
+            val aggnonce = Secp256k1.musigNonceAgg(pubnonces.toTypedArray())
+
+            val keyaggCaches = (0 until 2).map { ByteArray(Secp256k1.MUSIG2_PUBLIC_KEYAGG_CACHE_SIZE) }
+            val aggpubkey = Secp256k1.musigPubkeyAgg(pubkeys.toTypedArray(), keyaggCaches[0])
+            assertContentEquals(aggpubkey, Secp256k1.musigPubkeyAgg(pubkeys.toTypedArray(), keyaggCaches[1]))
+            assertContentEquals(keyaggCaches[0], keyaggCaches[1])
+            val msg32 = Hex.decode("0303030303030303030303030303030303030303030303030303030303030303")
+            val sessions = (0 until 2).map { Secp256k1.musigNonceProcess(aggnonce, msg32, keyaggCaches[it]) }
+
+            // we sign with the wrong secret nonce. it should fail (i.e. trigger an exception) but not crash the JVM
+            Secp256k1.musigPartialSign(secnonces[1], privkeys[0], keyaggCaches[0], sessions[0])
+        }
     }
 
     @Test