ObjectMonitor.object() can be null so we need to defend against it. This bug was discovered by code inspection while working on [JDK-8280555](https://bugs.openjdk.org/browse/JDK-8280555). We have no test for this, and I'm not sure how to reproduce this with HSDB like the [JDK-8280555](https://bugs.openjdk.org/browse/JDK-8280555) test did. I did at least verify that the HSDB "Monitor Cache Dump" feature still works after this fix, although none of the monitors had a null object.
------------- Commit messages: - Check for null mon.object() Changes: https://git.openjdk.org/jdk/pull/15369/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15369&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8280743 Stats: 6 lines in 1 file changed: 4 ins; 0 del; 2 mod Patch: https://git.openjdk.org/jdk/pull/15369.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/15369/head:pull/15369 PR: https://git.openjdk.org/jdk/pull/15369
