Issue 1397 - FilterEngine::ForceUpdateCheck crashes when passed a 0 std::function

src/FilterEngine.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 481 matching lines @@
 }
 
 void FilterEngine::UpdateAvailable(
     FilterEngine::UpdateAvailableCallback callback, JsValueList& params)
 {
   if (params.size() >= 1 && !params[0]->IsNull())
     callback(params[0]->AsString());
 }
 
 void FilterEngine::ForceUpdateCheck(
-    FilterEngine::UpdateCheckDoneCallback callback)
+    const FilterEngine::UpdateCheckDoneCallback& callback)
 {
-  std::string eventName = "_updateCheckDone";
-  eventName += ++updateCheckId;
-
-  jsEngine->SetEventCallback(eventName, std::bind(&FilterEngine::UpdateCheckDone,
-      this, eventName, callback, std::placeholders::_1));
-
   JsValuePtr func = jsEngine->Evaluate("API.forceUpdateCheck");
   JsValueList params;
-  params.push_back(jsEngine->NewValue(eventName));
+  if (callback)
+  {
+    std::string eventName = "_updateCheckDone" + std::to_string(++updateCheckId);
+    jsEngine->SetEventCallback(eventName, std::bind(&FilterEngine::UpdateCheckDone,
+      this, eventName, callback, std::placeholders::_1));
+    params.push_back(jsEngine->NewValue(eventName));
+  }
   func->Call(params);
 }
 
 void FilterEngine::UpdateCheckDone(const std::string& eventName,
     FilterEngine::UpdateCheckDoneCallback callback, JsValueList& params)
 {
   jsEngine->RemoveEventCallback(eventName);
 
   std::string error(params.size() >= 1 && !params[0]->IsNull() ? params[0]->AsString() : "");
   callback(error);
@@ skipping 81 matching lines @@
     FilterPtr filter = GetWhitelistingFilter(currentUrl, contentTypeMask, parentUrl);
     if (filter)
     {
       return filter;
     }
     currentUrl = parentUrl;
   }
   while (urlIterator != documentUrls.end());
   return FilterPtr();
 }