[jdk17] RFR: 8269232: assert(!is_jweak(handle)) failed: wrong method for detroying jweak

[Chris Plummer]

Don't let node->strongRef go below 0, or we end up thinking we still have a strongref when it is a weakref. Note the fact that we are doing nesting counts is still a bug (and the main cause of this issue). However, the fix for that is more complex, and for 17 I would prefer to just apply this simpler fix. For addressing the bigger issue, I filed the following:

[JDK-8269542](https://bugs.openjdk.java.net/browse/JDK-8269542): JDWP: EnableCollection support is no longer spec compliant after [JDK-8255987](https://bugs.openjdk.java.net/browse/JDK-8255987)

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Commit messages:
 - Don't decrement strongCount if already 0

Changes: https://git.openjdk.java.net/jdk17/pull/165/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17&pr=165&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269232
  Stats: 9 lines in 2 files changed: 6 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk17/pull/165.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17 pull/165/head:pull/165

PR: https://git.openjdk.java.net/jdk17/pull/165