Fix "audio used before assigned" bug

[FelixDombek]

audio_from_file did not correctly handle the case where the file cannot be opened. This PR fixes e.g. the crash mentioned in TheAlly's Riffusion tutorial.

Description

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• which issue is fixed.
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Closes: # (issue)

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• New and existing unit tests pass locally with my changes

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[freddyaboulton]

Thank you @FelixDombek ! Will merge once CI passes

[abidlabs]

Thanks for the contribution to the library @FelixDombek!