Increase STACK_BUFF_ALLOC to avoid execution_constants test failure on the hpita2.


sql/mysql_priv.h:
  Fix for bug #21476: stack overflow crashes server; error-message stack reservation too small
    - due to the failing execution_constants test on the hpita2, 
      increase STACK_BUFF_ALLOC, that is used in the Item_func::fix_fields() to 
      allocate on the stack a "dummy" buffer large enough for the corresponding exec.