Fix division by zero when memory data is not available

[mqudsi]

The total memory values may be zero when bottom is run on an unsupported
(or not-fully-supported) platform.

The previous behavior resulted in a NaN value for the memory datapoints,
which was passed through to tui-rs which ultimately panicked when
attempting to graph the memory widget.

Description

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Issue

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
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Checklist

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• Change has been tested to work
• Areas your change affects has been linted using rustfmt
• Code has been self-reviewed
• Code has been tested and no new breakage is introduced
• Documentation has been added/updated if needed
• No merge conflicts arise from the change

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[ClementTsang]

LGTM, thanks!

[ClementTsang]

@all-contributors add @mqudsi for code

[allcontributors]

@ClementTsang

I've put up a pull request to add @mqudsi! 🎉