core: Fixing scheduler, so it doesn't crash when an event is cancelled from the event invocation

Change-Id: I219df36a4ed19d75e97e676bedeba023fd53326e
diff --git a/daemon/core/scheduler.hpp b/daemon/core/scheduler.hpp
index 4b2e2d4..a8e60bb 100644
--- a/daemon/core/scheduler.hpp
+++ b/daemon/core/scheduler.hpp
@@ -92,6 +92,8 @@
   EventQueue m_events;
   EventQueue::iterator m_scheduledEvent;
   boost::asio::monotonic_deadline_timer m_deadlineTimer;
+
+  bool m_isEventExecuting;
 };
 
 } // namespace nfd