RFR: Fix use of uninitialized double

[William Kemper]

Member field was not initialized in constructor

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Commit messages:
 - Fix use of uninitialized double

Changes: https://git.openjdk.org/shenandoah/pull/194/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=194&range=00
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/194.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/194/head:pull/194

PR: https://git.openjdk.org/shenandoah/pull/194