Fix for double-free (and possibly other memory-corruption related) crashes at logrotate time

this is a direct fix of the issue revealing the problem that leads to crash

verification done with a patched icinga2 where the execution-order of the code lines of counter-parts involved in re-incrementing/decrementing Timer:Ptr is forced to be the one that leads to the obeserverd segfaults

refs #6737

lib/base/timer.cpp

@@ -110,7 +110,7 @@ void Timer::UninitializeThread()
 void Timer::Call()
 {
 	try {
-		OnTimerExpired(Timer::Ptr(this));
+		OnTimerExpired(this);
 	} catch (...) {
 		InternalReschedule(true);
 
@@ -301,8 +301,6 @@ void Timer::TimerThreadProc()
 			continue;
 		}
 
-		Timer::Ptr ptimer = timer;
-
 		/* Remove the timer from the list so it doesn't get called again
 		 * until the current call is completed. */
 		l_Timers.erase(timer);
@@ -312,6 +310,6 @@ void Timer::TimerThreadProc()
 		lock.unlock();
 
 		/* Asynchronously call the timer. */
-		Utility::QueueAsyncCallback(std::bind(&Timer::Call, ptimer));
+		Utility::QueueAsyncCallback(std::bind(&Timer::Call, timer));
 	}
 }

lib/base/timer.hpp

@@ -39,7 +39,7 @@ public:
 	void Reschedule(double next = -1);
 	double GetNext() const;
 
-	boost::signals2::signal<void(const Timer::Ptr&)> OnTimerExpired;
+	boost::signals2::signal<void(const Timer * const&)> OnTimerExpired;
 
 private:
 	double m_Interval{0}; /**< The interval of the timer. */

test/base-timer.cpp

@@ -24,7 +24,7 @@ BOOST_AUTO_TEST_CASE(interval)
 
 int counter = 0;
 
-static void Callback(const Timer::Ptr&)
+static void Callback(const Timer * const&)
 {
 	counter++;
 }