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.
Cool! It might be worth expanding the comment to state why we need 2 outputs as well.

LGTM with those changes, btw