init user->user struct with 
thd->security_ctx->priv_user context
if user->user is not initializied

mysql-test/r/grant.result:
  test result
mysql-test/t/grant.test:
  test case
sql/set_var.cc:
  init user->user struct with 
  thd->security_ctx->priv_user context
  if user->user is not initializied