RFR: 8374343: Fix SIGSEGV when lib/modules is unreadable [v2]

[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.

Boris Ulasevich has updated the pull request incrementally with one additional commit since the last revision:

  Move runtime image check to early VM init
  
  Suggested-by: Ioi Lam

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Changes:
  - all: https://git.openjdk.org/jdk/pull/28982/files
  - new: https://git.openjdk.org/jdk/pull/28982/files/f3da8c82..8152b3b2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28982&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28982&range=00-01

  Stats: 10 lines in 2 files changed: 4 ins; 5 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