Integrated: 8317360: Missing null checks in JfrCheckpointManager and JfrStringPool initialization routines

[Markus Grönlund]

Greetings,

Adding missing null checks for initialization routines.

Testing: jdk_jfr

Thanks
Markus

-------------

Commit messages:
 - 8317360

Changes: https://git.openjdk.org/jdk/pull/16310/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16310&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317360
  Stats: 6 lines in 2 files changed: 6 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/16310.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16310/head:pull/16310

PR: https://git.openjdk.org/jdk/pull/16310