store: fix potential race.
store: fix potential race.

eleme/fork_captcha,dchest/captcha,gotomypc/captcha,msn217/captcha,rainyear/captcha,oblank/captcha,zj8487/captcha,ZiRo-/captcha,ZiRo-/captcha,thomasmodeneis/captcha,kaz8/go-captcha