RFR: 8255348: NPE in PKIXCertPathValidator event logging code

[Sean Coffey]

Correction of NPE and updating of test cases. Minor refactoring of test library also.

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Commit messages:
 - 8255348: NPE in PKIXCertPathValidator event logging code

Changes: https://git.openjdk.java.net/jdk/pull/2150/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2150&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255348
  Stats: 83 lines in 6 files changed: 60 ins; 2 del; 21 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2150.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2150/head:pull/2150

PR: https://git.openjdk.java.net/jdk/pull/2150