fix crash due to incorrect state setting
fix crash due to incorrect state setting

Summary: the crash is caused by the misuse of `state()` inside `setState`.
we couldn't set state for a Tko'ed box in `on_down`: i.e. it remains `kUp`.

we should directly use stats_.state.

thanks @pavlo-fb

Test Plan: unit test

Reviewed By: @pavlo-fb

Differential Revision: D1834157
facebook/mcrouter,nvaller/mcrouter,leitao/mcrouter,leitao/mcrouter,evertrue/mcrouter,tempbottle/mcrouter,tempbottle/mcrouter,nvaller/mcrouter,zhlong73/mcrouter,glensc/mcrouter,reddit/mcrouter,easyfmxu/mcrouter,seem-sky/mcrouter,is00hcw/mcrouter,easyfmxu/mcrouter,is00hcw/mcrouter,easyfmxu/mcrouter,glensc/mcrouter,leitao/mcrouter,seem-sky/mcrouter,apinski-cavium/mcrouter,facebook/mcrouter,apinski-cavium/mcrouter,tempbottle/mcrouter,glensc/mcrouter,yqzhang/mcrouter,synecdoche/mcrouter,evertrue/mcrouter,yqzhang/mcrouter,seem-sky/mcrouter,is00hcw/mcrouter,evertrue/mcrouter,apinski-cavium/mcrouter,yqzhang/mcrouter,nvaller/mcrouter,zhlong73/mcrouter,synecdoche/mcrouter,tempbottle/mcrouter,is00hcw/mcrouter,facebook/mcrouter,facebook/mcrouter,reddit/mcrouter,nvaller/mcrouter,evertrue/mcrouter,seem-sky/mcrouter,zhlong73/mcrouter,leitao/mcrouter,yqzhang/mcrouter,reddit/mcrouter,synecdoche/mcrouter,zhlong73/mcrouter,easyfmxu/mcrouter,synecdoche/mcrouter,reddit/mcrouter,apinski-cavium/mcrouter,glensc/mcrouter