RFR: 8346714: [ASAN] compressedKlass.cpp reported applying non-zero offset to null pointer [v4]

[SendaoYan]

> Hi all,
> CompressedKlassPointers::sanity_check_after_initialization() src/hotspot/share/oops/compressedKlass.cpp:104:38 reported runtime error: applying non-zero offset 4294967296 to null pointer by clang17 UndefinedBehaviorSanitizer.
> 
> The _base initial as nullptr in function CompressedKlassPointers::initialize(address addr, size_t len) shows as below. In C/C++, offsetting a null pointer is undefined behavior. This PR do not change the original logic but eliminate the undefined behaviour in code, the risk is low.
> 
> ```c++
>     address const end = addr + len;
>     if (end <= (address)unscaled_max) {
>       _base = nullptr;
>       _shift = 0;

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

  add function p2u and use function p2u instead of cast manually

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/22848/files
  - new: https://git.openjdk.org/jdk/pull/22848/files/86f00f87..f9b9bb3e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=22848&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=22848&range=02-03

  Stats: 6 lines in 2 files changed: 5 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/22848.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22848/head:pull/22848

PR: https://git.openjdk.org/jdk/pull/22848