RFR: 8355230: Crash in fuzzer tests: assert(n != nullptr) failed: must not be null

[Roland Westrelin]

During IGVN, `TypeNode::make_paths_from_here_dead()` follows data
nodes until a `Phi`. The `Region` input for the input that that logic
goes through to reach the `Phi` is `null` causing the crash. I propose
simply adding an extra check for that corner case.

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Commit messages:
 - test
 - fix

Changes: https://git.openjdk.org/jdk/pull/25268/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25268&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8355230
  Stats: 104 lines in 2 files changed: 103 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/25268.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25268/head:pull/25268

PR: https://git.openjdk.org/jdk/pull/25268