Fix #33 add index check

[iegik]

https://github.com/smontanari/code-forensics/pull/34/files#diff-373d3571ba0e76f47fbe7575c1161ec9L21

• at that line I've got index value -1. So I add chack for that.

Unfortunately, I still did not see any results of commit-message-analysis task

[smontanari]

the latest changes have improved the detection and handling of errors when initialising graphs. thank you for your suggestion anyway.