Noissue - Don't allow to have an empty event callback in JsEngine

src/JsEngine.cpp

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2016 Eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 99 matching lines @@
     filename);
   CheckTryCatch(tryCatch);
   v8::Local<v8::Value> result = script->Run();
   CheckTryCatch(tryCatch);
   return JsValuePtr(new JsValue(shared_from_this(), result));
 }
 
 void AdblockPlus::JsEngine::SetEventCallback(const std::string& eventName,
     AdblockPlus::JsEngine::EventCallback callback)
 {
+  if (!callback)
+  {
+    RemoveEventCallback(eventName);
+    return;
+  }
   std::lock_guard<std::mutex> lock(eventCallbacksMutex);
   eventCallbacks[eventName] = callback;
 }
 
 void AdblockPlus::JsEngine::RemoveEventCallback(const std::string& eventName)
 {
   std::lock_guard<std::mutex> lock(eventCallbacksMutex);
   eventCallbacks.erase(eventName);
 }
 
@@ skipping 143 matching lines @@
 
 
 void AdblockPlus::JsEngine::SetGlobalProperty(const std::string& name, 
                                               AdblockPlus::JsValuePtr value)
 {
   auto global = GetGlobalObject();
   if (!global)
     throw std::runtime_error("Global object cannot be null");
   global->SetProperty(name, value);
 }