- Set the value of mp->thread before mp->count to avoid race condition.


mysys/thr_mutex.c:
  Set mp->thread before mp->count is changed. 
  Since mp->count is used in the macros 'safe_mutex_assert_not_owner' and 
  'safe_mutex_assert_owner' as an inidicator wheter to look at the mp->thread
  variable or not. This means that when mp->count is changed, the mp->thread
  should akready be set to the owning thread.
  Also set mp->thread to 0 when unlocking the mutex.