message: use is_null() to detect a null pointer
diff: use alloc::boxed::Box;
  use arch::percore::*;
  -use core::{mem, ptr};
  +use core::mem;
  use scheduler;
  use scheduler::task::PriorityTaskQueue;
  @@ -34,7 +34,7 @@ impl Drop for CondQueue {
  #[no_mangle]
  pub unsafe fn sys_destroy_queue(ptr: usize) -> i32 {
  let id = ptr as *mut usize;
  - if id == ptr::null_mut() {
  + if id.is_null() {
  debug!("sys_wait: ivalid address to condition variable");
  return -1;
  }
  @@ -53,7 +53,7 @@ pub unsafe fn sys_destroy_queue(ptr: usize) -> i32 {
  #[no_mangle]
  pub unsafe fn sys_notify(ptr: usize, count: i32) -> i32 {
  let id = ptr as *const usize;
  - if id == ptr::null() || *id == 0 {
  + if id.is_null() || *id == 0 {
  // invalid argument
  debug!("sys_notify: invalid address to condition variable");
  return -1;
  @@ -85,7 +85,7 @@ pub unsafe fn sys_notify(ptr: usize, count: i32) -> i32 {
  #[no_mangle]
  pub unsafe fn sys_wait(ptr: usize, timeout_ns: i64) -> i32 {
  let id = ptr as *mut usize;
  - if id == ptr::null_mut() {
  + if id.is_null() {
  debug!("sys_wait: ivalid address to condition variable");
  return -1;
  }
  