kernel: Fix kernel panic when adding a timer before another one

This was caused because add_to_timer_queue() did not set active_clock when inserting a timer before the end, making disarm() a no-op.
Therefore, the clock would continue to use the timer after it had been freed.

kernel/src/thread/Clock.cpp

@@ -79,6 +79,7 @@ void Clock::add_to_timer_queue(Timer* timer)
         {
             t->delta_ticks -= timer->delta_ticks;
             m_timer_queue.add_before(t, timer);
+            timer->active_clock = this;
             return;
         }
         timer->delta_ticks -= t->delta_ticks;