Prevent memory leaks when adding listeners

Check if the callback has already been added to the set before protecting it. Vice versa for unprotecting it when removing the listener.

src/RJSRealm.mm

@@ -49,13 +49,17 @@ public:
     }
 
     void add_notification(JSObjectRef notification) {
+        if (!m_notifications.count(notification)) {
             JSValueProtect(m_context, notification);
             m_notifications.insert(notification);
         }
+    }
     void remove_notification(JSObjectRef notification) {
+        if (m_notifications.count(notification)) {
             JSValueUnprotect(m_context, notification);
             m_notifications.erase(notification);
         }
+    }
     void remove_all_notifications() {
         for (auto notification : m_notifications) {
             JSValueUnprotect(m_context, notification);