Fix message offset

[vkresch]

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Resolves #372.

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This PR fixes the appending of int values to the message_offsets list. I also add a destructor method to close the opened file correctly after usage.

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@jdsika review and merge. Thanks.

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• My code and comments follow the style guidelines and contributors guidelines of this project.
• I have performed a self-review of my own code.
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• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests / travis ci pass locally with my changes.