Small fix with not matching types

src/kernel/synch.rs

@@ -24,6 +24,11 @@ pub struct Semaphore {
 
 impl Semaphore {
 
+    ///     Initializes a semaphore, so that it can be used for synchronization.
+    ///
+    ///  ### Parameters
+    /// - *counter* initial value of counter
+    /// - *thread_manager* Thread manager which managing threads
     pub fn new(counter: i32, thread_manager: Rc<RefCell<ThreadManager>>) -> Semaphore{
         Semaphore { counter, waiting_queue: List::new(), thread_manager}
     }
@@ -153,7 +158,10 @@ impl Lock {
     }
 
     pub fn held_by_current_thread(&mut self, current_thread: Rc<RefCell<Thread>>) -> bool {
-        Rc::ptr_eq(&self.owner, &current_thread)
+        match &self.owner {
+            Some(x) => Rc::ptr_eq(&x, &current_thread),
+            None => false
+        }
     }
 }