RFR: Avoid divide by zero error, improve variable names

[William Kemper]

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.

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Commit messages:
 - Avoid divide by zero error, improve variable names

Changes: https://git.openjdk.org/shenandoah/pull/188/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=188&range=00
  Stats: 12 lines in 1 file changed: 6 ins; 1 del; 5 mod
  Patch: https://git.openjdk.org/shenandoah/pull/188.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/188/head:pull/188

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