[xhash] fix a bug in xh_destroy
[xhash] fix a bug in xh_destroy

leavesbnw/picrin,picrin-scheme/picrin,ktakashi/picrin,dcurrie/picrin,picrin-scheme/picrin,ktakashi/picrin,leavesbnw/picrin,omasanori/picrin,dcurrie/picrin,leavesbnw/picrin,koba-e964/picrin,koba-e964/picrin,ktakashi/picrin,omasanori/picrin,koba-e964/picrin