Fix Bug in fd_get_file: In case fd not found, then returns NULL, w/ E

src/userprog/syscall.c

@@ -340,6 +340,9 @@ fd_get_file (int fd)
   struct hash_elem *e
     = hash_find (&thread_current ()->open_files, &fake_file_info.elem);
 
+  if (e == NULL)
+      return NULL;
+
   return hash_entry (e, struct open_file, elem);
 }