We should not assume that "thd" argument of reload_acl_and_cache() is
non-zero. Failure to do so will cause server to crash when one sends
SIGHUP to it.


sql/sql_parse.cc:
  reload_acl_and_cache():
    When we call this function from SIGHUP handler we pass 0 as "thd" argument
    to it. So we should not assume that thd is non-zero in it.