RFR: 8371574: Improve code coverage of PEM classes decode functionality

[Mikhail Yankelevich]

Missing code coverage for PEM API preview 2

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Commit messages:
 - JDK-8371574: Improve code coverage of PEM classes decode functionality

Changes: https://git.openjdk.org/jdk/pull/28368/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28368&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371574
  Stats: 10 lines in 1 file changed: 10 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/28368.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28368/head:pull/28368

PR: https://git.openjdk.org/jdk/pull/28368