Fix C code to satisfy sanitizers

[alexanderlaw]

No description provided.

[alexanderlaw]

Before enabling sanitizers, fix existing C code to avoid runtime failures during testing.

The code in CopyXLogRecordToWAL is not neon-specific, but the usage is (see neondatabase/postgres#554 (comment)).

This pull request supersedes neondatabase/postgres#554, where a change in postgres code was discussed, and replaces a part of #10401 (as of 2025-01-22), where a change in walproposer, in particular, was approved.

[github-actions]

7403 tests run: 7015 passed, 1 failed, 387 skipped (full report)

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Failures on Postgres 15

• test_metrics_normal_work: release-x86-64-with-lfc

# Run all failed tests locally:
scripts/pytest -vv -n $(nproc) -k "test_metrics_normal_work[release-pg15]"

Flaky tests (4)

Postgres 17

• test_metrics_normal_work: release-x86-64-without-lfc

Postgres 16

• test_metrics_normal_work: release-x86-64-with-lfc

Postgres 15

• test_pageserver_gc_compaction_smoke[with_branches]: release-x86-64-with-lfc

Postgres 14

• test_timeline_copy[100000]: release-arm64-with-lfc

Test coverage report is not available

_{The comment gets automatically updated with the latest test results
e7cc860 at 2025-01-27T15:33:06.497Z :recycle:}