Bug in optimize

[emricksinisonos]

I have an issue when I run the optimize algorithm on this FST.

The algorithm panics because of an unwrap() occurring in the decode function that happens just after a minimize. More exactly, decode(tr.ilabel) is called with an ilabel == 0 which implies calling a get on a negative value here.

The issue seems to be introduced by this PR #166 as the algorithm doesn't panic in the latest commit before that change (8f7f486).