Fix confusion between 1-based Buffer and 0-based index

[hlinnaka]

The code was working correctly, but was incorrectly using Buffer for a 0-based index into the BufferDesc array.

[github-actions]

3096 tests run: 2969 passed, 0 failed, 127 skipped (full report)

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Flaky tests (3)

Postgres 16

• test_crafted_wal_end[last_wal_record_xlog_switch_ends_on_page_boundary]: release

Postgres 15

• test_ancestor_detach_reparents_earlier: debug

Postgres 14

• test_download_remote_layers_api: debug

Code coverage* (full report)

• functions: 31.3% (6414 of 20481 functions)
• lines: 48.0% (49312 of 102647 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
0aa9549 at 2024-05-22T12:38:29.056Z :recycle:}

[hlinnaka]

Was committed as commit 3404e76