redis-benchmark: add auth option

Closes #1097
redis-benchmark: Remove unused buffer

This just deletes old code that didn't get removed when
logic changed.  We were setting offsets that never
got read anywhere.

Since clients are now just cloned, we don't need to track
per-client buffer offsets anywhere because they are all
the same from the original client.
Fix potential error when generating INFO string

Clang (scan-build) flags this as an error, but it can't
happen under normal usage because the input is verified
in caller function.  Clang doesn't trust the caller
to always pass correct input though, so it gets registered
as an error in the report.

It's better to remove the possibility of a crash, even if
only technically.  It also makes our error report cleaner
since we know this isn't a problem.