Fix race-condition in process result (memory leak), fix infinite loop in donors_list

src/userprog/process.c

@@ -262,11 +262,12 @@ process_exit (void)
          struct process_result *, and may be waiting so call sema_up */
       else
         {
-          lock_release (&cur->result->lock);
           sema_up (&cur->result->sema);
+          lock_release (&cur->result->lock);
         }
     }
 
+  /* Free child process results or signal parent's death. */
   struct list_elem *e;
   for (e = list_begin (&cur->child_results);
        e != list_end (&cur->child_results); e = list_next (e))
@@ -283,8 +284,8 @@ process_exit (void)
       /* Child is still alive, signal via sema that parent has died. */
       else
         {
-          lock_release (&result->lock);
           sema_up (&result->sema);
+          lock_release (&result->lock);
         }
     }