[jdk16] RFR: 8257521: runtime/logging/MonitorInflationTest.java crashed in MonitorList::unlink_deflated

[Daniel D.Daugherty]

This is a trivial fix to MonitorList::unlink_deflated() to prevent it from
crashing when called with an empty in-use MonitorList. That condition
can only arise during the final audit during VM shutdown.

This fix has been tested with runtime/logging/MonitorInflationTest.java
combined with temporary debug code that delays the background threads
that normally call Object.wait() very early in the VM's life. By delaying those
background threads, we increase the likelihood of having an empty in-use
MonitorList.

The fix has also been tested with Mach5 Tier[1-3].

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Commit messages:
 - 8257521

Changes: https://git.openjdk.java.net/jdk16/pull/62/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk16&pr=62&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257521
  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk16/pull/62.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16 pull/62/head:pull/62

PR: https://git.openjdk.java.net/jdk16/pull/62