[BUG] AttributeSet::Descriptor::{read,write} has out of bounds memory access when size_t is not 64-bit

Environment

Operating System: alpine linux 32-bit
Version / Commit SHA: 402b91251cca6d8b46a70205086b923f85bd4814
Other: https://gitlab.alpinelinux.org/sertonix/aports/-/merge_requests/141

Describe the bug

(A clear and concise description of what the bug is.)

In openvdb/openvdb/points/AttributeSet.cc there are 4 calls to reinterpret_cast used to read/write data followed by a sizeof(Index64) for the length even though the actual data is of type size_t. On architectures where size_t is not 64-bit this results in out of bounds memory access. There should be something like sizeof(size_t) or sizeof(it->second) used instead.

To Reproduce

Steps to reproduce the behavior:

1. Run tests on an alpine linux 32-bit system
2. The test suite crashes due to stack corruption

Expected behavior

The test suite should not crash and mostly pass.

Additional context

I made a patch which I don't mind being committed with changed authorship but have no interest in signing the CLA: https://github.com/AcademySoftwareFoundation/openvdb/pull/2173

Inspect the four reinterpret_cast calls in openvdb/openvdb/points/AttributeSet.cc that use sizeof(Index64) for size_t data. Run the test suite on an Alpine Linux 32-bit environment and verify it no longer crashes from stack corruption. Done means the accesses use the actual data size and the tests mostly pass on 32-bit systems.