[jdk25u-dev] 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 

Results: 
[hotspot_compiler_report.html](https://github.com/user-attachments/files/25494824/hotspot_compiler_report.html)
[TestUnlockNodeNullMemprof_report.html](https://github.com/user-attachments/files/25494831/TestUnlockNodeNullMemprof_report.html)

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Commit messages:
 - Backport 7c6c34e150cf01cec5d166f6cbb8a649c75b0627

Changes: https://git.openjdk.org/jdk25u-dev/pull/304/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk25u-dev&pr=304&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/jdk25u-dev/pull/304.diff
  Fetch: git fetch https://git.openjdk.org/jdk25u-dev.git pull/304/head:pull/304

PR: https://git.openjdk.org/jdk25u-dev/pull/304