Fix off-by-one error

The precondition check in rollBackLastOutput should make sure there are
two or more outputs in order to proceed.

Extended test checking the error path to zero and one output and
modified a test that had only one output in order to make it satisfy the
precondition.
Ok, so this would be an error that should never happen as the callsite
will be doing the same check, right?

I wonder if we should use a more specific error to make it clear that if
it's triggered, we certainly do have a bug somewhere.