[commissioner] use strnlen instead of strlen (#4404)
[commissioner] use strnlen instead of strlen (#4404)

srickardti/openthread,lanyuwen/openthread,librasungirl/openthread,abtink/openthread,bukepo/openthread,librasungirl/openthread,openthread/openthread,bukepo/openthread,openthread/openthread,srickardti/openthread,chshu/openthread,lanyuwen/openthread,lanyuwen/openthread,chshu/openthread,chshu/openthread,abtink/openthread,lanyuwen/openthread,jwhui/openthread,bukepo/openthread,lanyuwen/openthread,jwhui/openthread,chshu/openthread,librasungirl/openthread,openthread/openthread,chshu/openthread,bukepo/openthread,jwhui/openthread,chshu/openthread,abtink/openthread,librasungirl/openthread,abtink/openthread,lanyuwen/openthread,jwhui/openthread,srickardti/openthread,srickardti/openthread,openthread/openthread