RFR: Avoid divide by zero error, improve variable names

[Kelvin Nilsen]

On Wed, 21 Dec 2022 22:19:13 GMT, William Kemper <wkemper at openjdk.org> wrote:

> Depending on when the periodic thread runs the accounting task and how much CPU time the process receives, we may see a very small elapsed process time. Such a time should not be used to compute MMU.

Marked as reviewed by kdnilsen (Committer).

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PR: https://git.openjdk.org/shenandoah/pull/188