RFR: Initialize promotion failed member to false

[William Kemper]

Trivial fix

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Commit messages:
 - Initialize promotion failed member to false

Changes: https://git.openjdk.java.net/shenandoah/pull/92/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=92&range=00
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/92.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/92/head:pull/92

PR: https://git.openjdk.java.net/shenandoah/pull/92