redis-cli: stop showing incorrectly selected DB
redis-cli: stop showing incorrectly selected DB

Previously redis-cli would happily show "-1" or "99999"
as valid DB choices.

Now, if the SELECT call returned an error, we don't update
the DB number in the CLI.

Inspired by @anupshendkar in #1313

Fixes #566, #1313

JackieXie168/redis,JackieXie168/redis,JackieXie168/redis,JackieXie168/redis