Fix small memory accounting bug in libpagestore

[MMeent]

Found while searching for other issues in shared memory.

The bug should be benign, in that it over-allocates memory for this struct, but doesn't allow for out-of-bounds writes.

Problem

We over-allocate memory in libpagestore's shared memory area

Summary of changes

We fix the memory accounting error

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[hlinnaka]

fixes #9222

[github-actions]

5022 tests run: 4864 passed, 0 failed, 158 skipped (full report)

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Flaky tests (1)

Postgres 14

• test_subscriber_restart: release-x86-64

Test coverage report is not available

_{The comment gets automatically updated with the latest test results
35d57f7 at 2024-10-01T16:31:38.383Z :recycle:}