Refactor derivation for unwrap safety

Author: wvanlint

src/headers/lnurl_auth_jwt.rs

@@ -172,8 +172,11 @@ fn linking_key_path(hashing_key: &PrivateKey, domain_name: &str) -> Result<Deriv
 	let mut engine = HmacEngine::<sha256::Hash>::new(&hashing_key.inner[..]);
 	engine.input(domain_name.as_bytes());
 	let result = Hmac::<sha256::Hash>::from_engine(engine).to_byte_array();
-	let children = (0..4)
-		.map(|i| u32::from_be_bytes(result[(i * 4)..((i + 1) * 4)].try_into().unwrap()))
+	// unwrap safety: We take 4-byte chunks, so TryInto for [u8; 4] never fails.
+	let children = result
+		.chunks_exact(4)
+		.take(4)
+		.map(|i| u32::from_be_bytes(i.try_into().unwrap()))
 		.map(ChildNumber::from);
 	Ok(DerivationPath::from_iter(children))
 }