fix(tracing/js): fault_fn not checked (#221)

fix a copy/paste error, checking `fault_fn` but used `result_fn`

Signed-off-by: jsvisa <delweng@gmail.com>

src/tracing/js/mod.rs

@@ -136,7 +136,7 @@ impl JsInspector {
             .as_object()
             .cloned()
             .ok_or(JsInspectorError::FaultFunctionMissing)?;
-        if !result_fn.is_callable() {
+        if !fault_fn.is_callable() {
             return Err(JsInspectorError::FaultFunctionMissing);
         }
 
@@ -648,4 +648,17 @@ mod tests {
         ));
         assert!(result.is_err());
     }
+
+    #[test]
+    fn test_fault_fn_not_callable() {
+        let code = r#"
+            {
+                result: function() {},
+                fault: {},
+            }
+        "#;
+        let config = serde_json::Value::Null;
+        let result = JsInspector::new(code.to_string(), config);
+        assert!(matches!(result, Err(JsInspectorError::FaultFunctionMissing)));
+    }
 }