RFR: 8331626: unsafe.cpp:162:38: runtime error in index_oop_from_field_offset_long - applying non-zero offset 4563897424 to null pointer [v2]

[Martin Doerr]

> `index_oop_from_field_offset_long` is sometimes used to access an absolute address by using `p == nullptr`. Unfortunately, `nullptr + byte_offset` implies undefined behavior and should better get fixed. UBSan complains about it (see JBS issue).
> A possible solution is to replace pointer arithmetic by integer arithmetic. We can use unsigned because `assert_field_offset_sane` checks that `byte_offset >= 0`.

Martin Doerr has updated the pull request incrementally with one additional commit since the last revision:

  Change coding style.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/19087/files
  - new: https://git.openjdk.org/jdk/pull/19087/files/24ca3361..c8bc69b9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19087&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19087&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/19087.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19087/head:pull/19087

PR: https://git.openjdk.org/jdk/pull/19087