RFR: 8369642: [ubsan] nmethod::nmethod null pointer passed as argument 2 to memcpy [v2]

[Chad Rakoczy]

> [JDK-8316694](https://bugs.openjdk.org/browse/JDK-8316694) introduced a counter so that the nmethod immutable data can be shared between relocated nmethods to eliminate an unnecessary copy. The counter is aligned in memory so that must be taken into account when calculating the amount of memory used by the counter

Chad Rakoczy has updated the pull request incrementally with one additional commit since the last revision:

  Add reference counter offset

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/27778/files
  - new: https://git.openjdk.org/jdk/pull/27778/files/64e9be35..2827dca8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27778&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27778&range=00-01

  Stats: 30 lines in 2 files changed: 14 ins; 10 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/27778.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27778/head:pull/27778

PR: https://git.openjdk.org/jdk/pull/27778