RFR: 8349888: AOTMode=create crashes in AOTClassLinker::class_category_name

[Aleksey Shipilev]

Reliably reproduces in current mainline, see the bug for reproducer. I think we are crashing after feeding `nullptr` to `class_category_name`. That `nullptr` is from `ArchiveBuilder::current()->get_buffered_addr` of `HelloStream`, which is skipped, as per warning message right before the crash. 

So it looks like the fix is to check if we are dealing with class that is available in the buffer before reaching for its buffered address. @iklam, see if this is a right fix?

Additional testing:
 - [x] Linux x86_64 server fastdebug, reproducer now passes
 - [x] Linux x86_64 server fastdebug, `runtime/cds` passes

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/23581/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23581&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349888
  Stats: 5 lines in 1 file changed: 3 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/23581.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23581/head:pull/23581

PR: https://git.openjdk.org/jdk/pull/23581