[jdk11u-dev] RFR: 8331863: DUIterator_Fast used before it is constructed

[Radek Čáp]

Backport of JDK-8331863 from 17u-dev.

Clean backport. Passed tier1 tests. Passed gtests.

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Commit messages:
 - Backport ed30b9c1452042ed94fc7aa5a18e2a647788e2ae

Changes: https://git.openjdk.org/jdk11u-dev/pull/2973/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2973&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331863
  Stats: 2 lines in 1 file changed: 1 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/2973.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2973/head:pull/2973

PR: https://git.openjdk.org/jdk11u-dev/pull/2973