refactor: remove un-tested early returns

Replace early returns in KeyPair::KeyPair() with asserts.

The if statements imply there is an error we are handling, but keypair_xonly_pub
and xonly_pubkey_serialize can only fail if the keypair object is malformed, i.e.,
it was created with a bad secret key. Since we check that the keypair was created
successfully before attempting to extract the public key, using asserts more
accurately documents what we expect here and removes untested branches from the code.

src/key.cpp

@@ -414,9 +414,9 @@ KeyPair::KeyPair(const CKey& key, const uint256* merkle_root)
     bool success = secp256k1_keypair_create(secp256k1_context_sign, keypair, UCharCast(key.data()));
     if (success && merkle_root) {
         secp256k1_xonly_pubkey pubkey;
-        if (!secp256k1_keypair_xonly_pub(secp256k1_context_sign, &pubkey, nullptr, keypair)) return;
         unsigned char pubkey_bytes[32];
-        if (!secp256k1_xonly_pubkey_serialize(secp256k1_context_sign, pubkey_bytes, &pubkey)) return;
+        assert(secp256k1_keypair_xonly_pub(secp256k1_context_sign, &pubkey, nullptr, keypair));
+        assert(secp256k1_xonly_pubkey_serialize(secp256k1_context_sign, pubkey_bytes, &pubkey));
         uint256 tweak = XOnlyPubKey(pubkey_bytes).ComputeTapTweakHash(merkle_root->IsNull() ? nullptr : merkle_root);
         success = secp256k1_keypair_xonly_tweak_add(secp256k1_context_static, keypair, tweak.data());
     }