[jdk26u] RFR: 8370502: C2: segfault while adding node to IGVN worklist

[Ozan Çetin]

Backporting JDK-8370502: C2: segfault while adding node to IGVN worklist.

This PR backports a fix for a C2 segmentation fault that occurs when expanding an UnlockNode with a null memory projection. The fix adds a necessary null check to ensure fallthrough_memproj is valid before attempting to replace it in the IGVN worklist.

make run-test TEST=tier1
make run-test TEST=hotspot_compiler
make run-test TEST=test/hotspot/jtreg/compiler/c2/TestUnlockNodeNullMemprof.java

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Commit messages:
 - Backport 7c6c34e150cf01cec5d166f6cbb8a649c75b0627

Changes: https://git.openjdk.org/jdk26u/pull/75/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk26u&pr=75&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370502
  Stats: 68 lines in 2 files changed: 56 ins; 5 del; 7 mod
  Patch: https://git.openjdk.org/jdk26u/pull/75.diff
  Fetch: git fetch https://git.openjdk.org/jdk26u.git pull/75/head:pull/75

PR: https://git.openjdk.org/jdk26u/pull/75