Fix a bug in srandmemberWithCountCommand()

In CASE 2, the call sunionDiffGenericCommand will involve the string "srandmember" 
> sadd foo one
(integer 1)
> sadd srandmember two
(integer 2)
> srandmember foo 3
1)"one"
2)"two"
Merge pull request #929 from ltnwgl/unstable

Fix a bug in srandmemberWithCountCommand()