[jdk11u-dev] RFR: 8282306: os::is_first_C_frame(frame*) crashes on invalid link access

[Johannes Bechberger]

Hardening os::is_first_C_frame(frame*) against accesses on invalid links.

Applies relatively clean (only a comment changed which hindered it from applying directly).

Tested via nightly SAP builds and on my own linux machine.

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Commit messages:
 - Backport 999da9bfc5be703141cdc07af455b4b6b2cc1aae

Changes: https://git.openjdk.org/jdk11u-dev/pull/1373/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=1373&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8282306
  Stats: 86 lines in 11 files changed: 60 ins; 20 del; 6 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/1373.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev pull/1373/head:pull/1373

PR: https://git.openjdk.org/jdk11u-dev/pull/1373