Fixed duration in meta data when generating subclips.

[hlzl]

Fixes issue #1316

• I have provided code that clearly demonstrates the bug and that only works correctly when applying this fix
• I have added suitable tests demonstrating a fixed bug or new/changed feature to the test suite in tests/
• I have properly documented new or changed features in the documentation or in the docstrings
• I have properly explained unusual or unexpected code in the comments around it
• I have formatted my code using black -t py36

[coveralls]

Coverage remained the same at 67.544% when pulling 947647c on hlzl:master into f19f189 on Zulko:master.

[tburrows13]

Thank you very much for this fix. I'll probably check and merge it sometime this week. Don't worry about the failing checks... they aren't your fault!