RFR: 8280743: HSDB "Monitor Cache Dump" command might throw NPE [v2]

[Chris Plummer]

> 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.

Chris Plummer has updated the pull request incrementally with one additional commit since the last revision:

  Move fetching the Oop to inside the else block.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/15369/files
  - new: https://git.openjdk.org/jdk/pull/15369/files/a3f3f0e9..410bce0c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15369&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15369&range=00-01

  Stats: 2 lines in 1 file changed: 1 ins; 1 del; 0 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