tttool explain: Do not trip over segments of length 0
tttool explain: Do not trip over segments of length 0

This fixes #117.

entropia/tip-toi-reveng,colinba/tip-toi-reveng,colinba/tip-toi-reveng,entropia/tip-toi-reveng,entropia/tip-toi-reveng,colinba/tip-toi-reveng,entropia/tip-toi-reveng,entropia/tip-toi-reveng,colinba/tip-toi-reveng,entropia/tip-toi-reveng