[jdk21u] RFR: 8316436: ContinuationWrapper uses unhandled nullptr oop

[Aleksey Shipilev]

Clean backport to improve Loom code reliability.

Additional testing:
 - [x] MacOS AArch64 server fastdebug, `jdk_loom hotspot_loom`
 - [ ] Linux x86_64 server fastdebug, `tier1 tier2 tier3`

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Commit messages:
 - Backport 292aad2c4901f2ffba37274763e1cc617711918e

Changes: https://git.openjdk.org/jdk21u/pull/346/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=346&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316436
  Stats: 28 lines in 2 files changed: 8 ins; 12 del; 8 mod
  Patch: https://git.openjdk.org/jdk21u/pull/346.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/346/head:pull/346

PR: https://git.openjdk.org/jdk21u/pull/346