RFR: 8351333: [ubsan] CDSMapLogger::log_region applying non-zero offset to null pointer

[Ioi Lam]

Please review this trivial fix for UBSAN: avoid doing address calculating with nullptr as the base.

-------------

Commit messages:
 - 8351333: [ubsan] CDSMapLogger::log_region applying non-zero offset to null pointer

Changes: https://git.openjdk.org/jdk/pull/24880/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24880&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8351333
  Stats: 6 lines in 1 file changed: 4 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/24880.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24880/head:pull/24880

PR: https://git.openjdk.org/jdk/pull/24880