fix issue 8439: bonus3 baddmonsterdropitem not specified on one item

sd->add_drop[i].race and sd->add_drop[i].class_ can be -1 here, which causes any item drop to pass this - see http://rathena.org/board/tracker/issue-8439-bonus3-baddmonsterdropitem-not-specified-on-one-item%EF%BC%81/