RFR: 8357576: FieldInfo::_index is not initialized by the constructor

[Matias Saavedra Silva]

FieldInfo::_index is not initialized in either of the FieldInfo constructors so this patch adds initialization to both constructors. Verified with tier 1-5 tests

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Commit messages:
 - 8357576: FieldInfo::_index is not initialized by the constructor

Changes: https://git.openjdk.org/jdk/pull/25554/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25554&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357576
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/25554.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25554/head:pull/25554

PR: https://git.openjdk.org/jdk/pull/25554