RFR: 8374343: Fix SIGSEGV when lib/modules is unreadable

[Boris Ulasevich]

The JVM crashes with a SIGSEGV during startup if the runtime image (lib/modules) exists but is unreadable. In this scenario, ClassLoader::get_jrt_entry() returns nullptr, leading to a crash. This change adds a null check to avoid the dereference and bail out gracefully.

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Commit messages:
 - 8374343: Fix SIGSEGV when lib/modules is unreadable

Changes: https://git.openjdk.org/jdk/pull/28982/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28982&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8374343
  Stats: 6 lines in 1 file changed: 5 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/28982.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28982/head:pull/28982

PR: https://git.openjdk.org/jdk/pull/28982