Check the length of timers before delete

kernel/aster-nix/src/process/process/timer_manager.rs

@@ -198,11 +198,15 @@ impl PosixTimerManager {
     /// Removes the POSIX timer with the ID `timer_id`.
     pub fn remove_posix_timer(&self, timer_id: usize) -> Option<Arc<Timer>> {
         let mut timers = self.posix_timers.lock();
-        let timer = timers[timer_id].take();
-        if timer.is_some() {
-            // Holding the lock of `posix_timers` is required to operate the `id_allocator`.
-            self.id_allocator.lock().free(timer_id);
+        if timer_id < timers.len() {
+            let timer = timers[timer_id].take();
+            if timer.is_some() {
+                // Holding the lock of `posix_timers` is required to operate the `id_allocator`.
+                self.id_allocator.lock().free(timer_id);
+            }
+            timer
+        } else {
+            None
         }
-        timer
     }
 }