Issue 5116 - Copy the timer delay before waiting for it.

src/DefaultTimer.cpp

 /*
 * This file is part of Adblock Plus <https://adblockplus.org/>,
 * Copyright (C) 2006-2017 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 45 matching lines @@
     std::unique_lock<std::mutex> lock(mutex);
     if (timers.empty())
     {
       conditionVariable.wait(lock, [this]()->bool
       {
         return shouldThreadStop || !timers.empty();
       });
     }
     else
     {
-      conditionVariable.wait_until(lock, timers.top().fireAt);
+      auto fireAt = timers.top().fireAt;
+      conditionVariable.wait_until(lock, fireAt);
     }
     // execute all expired timers and remove them
     while (!shouldThreadStop && !timers.empty() && timers.top().fireAt <= std::chrono::steady_clock::now())
     {
       auto callback = timers.top().callback;
       timers.pop();
       // allow to put new timers while this timer is being processed
       lock.unlock();
       try
       {
         callback();
       }
       catch (...)
       {
         // do nothing, but the thread will be alive.
       }
       lock.lock();
     }
     if (shouldThreadStop)
       return;
   }
 }