RFR: 8352112: [ubsan] hotspot/share/code/relocInfo.cpp:130:37: runtime error: applying non-zero offset 18446744073709551614 to null pointer [v2]

[Vladimir Kozlov]

> Before [JDK-8343789](https://bugs.openjdk.org/browse/JDK-8343789) `relocation_begin()` was never null even when there was no relocations - it pointed to the beginning of constant or code section in such case. It was used by relocation code to simplify code and avoid null checks.
> With that fix `relocation_begin()` points to address in `CodeBlob::_mutable_data` field which could be `nullptr` if there is no relocation and metadata.
> 
> There easy fix is to avoid `nullptr` in `CodeBlob::_mutable_data`. We could do that similar to what we do for `nmethod::_immutable_data`: [nmethod.cpp#L1514](https://github.com/openjdk/jdk/blob/master/src/hotspot/share/code/nmethod.cpp#L1514).
> 
> Tested tier1-4, stress, xcomp. Verified with failed tests listed in bug report.

Vladimir Kozlov has updated the pull request incrementally with one additional commit since the last revision:

  Update field default setting

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/24102/files
  - new: https://git.openjdk.org/jdk/pull/24102/files/d8b8cf31..cc14ad2a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24102&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24102&range=00-01

  Stats: 5 lines in 1 file changed: 1 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/24102.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24102/head:pull/24102

PR: https://git.openjdk.org/jdk/pull/24102