RFR: 8354522: Clones of DecimalFormat cause interferences when used concurrently

[Johannes Graham]

The `DigitList` class used in `DecimalFormat` does not reset the `data` array in its clone method. This can cause interference when clones are used concurrently.

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Commit messages:
 - reset "data" field in clone

Changes: https://git.openjdk.org/jdk/pull/24598/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24598&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8354522
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/24598.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24598/head:pull/24598

PR: https://git.openjdk.org/jdk/pull/24598