message: remutex: use std::ptr::null_mut() for null ptr
diff: use std::cell::UnsafeCell;
  use std::mem;
  +use std::ptr;
  use std::sync::{self, MutexGuard, Mutex};
  use std::sync::atomic::{AtomicUsize, Ordering};
  use std::marker::PhantomData;
  @@ -67,7 +68,7 @@ impl RecursiveMutex {
  owner: AtomicUsize::new(0),
  recursion: UnsafeCell::new(0),
  mutex: Mutex::new(()),
  - guard: UnsafeCell::new(0 as *mut _),
  + guard: UnsafeCell::new(ptr::null_mut()),
  }
  }
  