The possibility of the race is removed by changing sequence of calls

  pthread_mutex_unlock(&mi->run_lock);
  pthread_cond_broadcast(&mi->stop_cond);  

into

  pthread_cond_broadcast(&mi->stop_cond);
  pthread_mutex_unlock(&mi->run_lock);

at the end of I/O thread (similar change at the end of SQL thread). This ensures 
that no thread waiting on the condition executes between the broadcast and the 
unlock and thus can't delete the mi structure which caused the bug.