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.
If we had only one then we should do an output split instead. So I check that we have more than one - and if not we return an error.