Fix the sleep queue even more

kernel/include/sleep_queue.hpp

@@ -135,6 +135,7 @@ public:
         // If the queue still contains our pid, it means a wake up
         // from timeout
 
+        if(!queue.empty()){
             // If the pid is on top, pop it
             if(queue.top() == pid){
                 obtained = false;
@@ -146,6 +147,7 @@ public:
                 obtained = false;
                 queue.replace(pid, scheduler::INVALID_PID);
             }
+        }
 
         // Final release of the lock
         lock.release();