issue #150 fix: return nil value if the set not exist for 'srandmember
issue #150 fix: return nil value if the set not exist for 'srandmember

bssrdf/ardb,bssrdf/ardb,spitfire2/ardb,cloudrain21/ardb,wfxiang08/ardb,tempbottle/ardb,spitfire2/ardb,yinqiwen/ardb,placrosse/ardb,tempbottle/ardb,placrosse/ardb,lsm/ardb,cloudrain21/ardb,yinqiwen/ardb,lsm/ardb,wfxiang08/ardb