RFR: 8318915: Enhance checks in BigDecimal.toPlainString()

[Raffaello Giulietti]

Prevent a `NegativeArraySizeException` in `BigDecimal.toPlainString()`, throwing `OutOfMemoryError` instead to indicate that the resulting `String` would be too large.

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Commit messages:
 - 8318915: Enhance checks in BigDecimal.toPlainString()

Changes: https://git.openjdk.org/jdk/pull/16457/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16457&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318915
  Stats: 56 lines in 2 files changed: 43 ins; 1 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/16457.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16457/head:pull/16457

PR: https://git.openjdk.org/jdk/pull/16457