Fix cli prompt for failed select command

Select command with negative integer as a input makes redis-cli to
refresh its prompt wrongly.

For example:
    [root@anup-centos-6 src]# ./redis-cli
    127.0.0.1:6379> select -1
    (error) ERR invalid DB index
    127.0.0.1:6379[-1]>

The patch fixes this problem.

Signed-off-by: Anup Shendkar <anupshendkar@gmail.com>