Add assert for correct state when removing timer

src/timers.c

@@ -76,6 +76,8 @@ void uc_timers_remove(struct UCTimers *timers, struct UCTimer *timer)
         timer->ready_entry.tqe_prev = (struct UCTimer **)105;
         return;
     }
+    assert(timer->state == UC_TIMER_ACTIVE ||
+           timer->state == UC_TIMER_PENDING); 
 
     uc_rb_remove(&timers->timer_tree, &timer->rb_node); //remove it, safe for the
                                                     //callback to re_add it