fix: do not read extra block if offset = at+childSize

[hacdias]

Thanks to @aschmahmann for finding this bug. The issue is that if offset == at+childSize, we would be reading an unnecessary block.

Needed in the context of IPIP-402 (ipfs/boxo#303). We were getting one unnecessary block. I added a test, which fails without the fix.

[github-actions]

Suggested version: v1.7.1

Comparing to: v1.7.0 (diff)

Changes in go.mod file(s):

(empty)

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# summary
Suggested version: v1.7.1

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