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

include/AdblockPlus/FilterEngine.h

 /*
  * 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 450 matching lines @@
      * `FilterEngine` will automatically check for updates in regular intervals,
      * so applications should only call this when the user triggers an update
      * check manually.
      * @param callback Optional callback to invoke when the update check is
      *        finished. The string parameter will be empty when the update check
      *        succeeded, or contain an error message if it failed.
      *        Note that the callback will be invoked whether updates are
      *        available or not - to react to updates being available, use
      *        `FilterEngine::SetUpdateAvailableCallback()`.
      */
-    void ForceUpdateCheck(UpdateCheckDoneCallback callback);
+    void ForceUpdateCheck(const UpdateCheckDoneCallback& callback = UpdateCheckDoneCallback());
 
     /**
      * Sets the callback invoked when the filters change.
      * @param callback Callback to invoke.
      */
     void SetFilterChangeCallback(FilterChangeCallback callback);
 
     /**
      * Removes the callback invoked when the filters change.
      */
@@ skipping 60 matching lines @@
       const JsValueList& params);
     FilterPtr GetWhitelistingFilter(const std::string& url,
       ContentTypeMask contentTypeMask, const std::string& documentUrl) const;
     FilterPtr GetWhitelistingFilter(const std::string& url,
       ContentTypeMask contentTypeMask,
       const std::vector<std::string>& documentUrls) const;
   };
 }
 
 #endif